Merge #733

733: Do not transform (parse) commands that fail inside of a pipeline r=badboy

Hii all!! 

First of all, thanks a bunch for this gem and happy new year 😄 .

I recently observed a exception triggered from another gem which extensively uses `redis-rb`[1]:
```ruby
undefined method `each_slice' for #<Redis::CommandError:0x007fc46d9ca958> (NoMethodError)
```
<details><summary>Full stacktrace</summary>

```
/Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis.rb:2809:in `block in <class:Redis>': undefined method `each_slice' for #<Redis::CommandError:0x007fc46d9ca958> (NoMethodError)
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/pipeline.rb:114:in `_set'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/pipeline.rb:61:in `block in finish'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/pipeline.rb:60:in `each'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/pipeline.rb:60:in `each_with_index'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/pipeline.rb:60:in `each'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/pipeline.rb:60:in `map'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/pipeline.rb:60:in `finish'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/client.rb:155:in `block in call_pipeline'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/client.rb:291:in `with_reconnect'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/client.rb:153:in `call_pipeline'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis.rb:2307:in `block in pipelined'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis.rb:45:in `block in synchronize'
	from /Users/javierhonduco/.rubies/ruby-2.4.0/lib/ruby/2.4.0/monitor.rb:214:in `mon_synchronize'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis.rb:45:in `synchronize'
	from /Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis.rb:2303:in `pipelined'
	[...]
```
</details>


I attempted getting a minimal reproduction and ended up with:
```ruby
require 'redis'
r = Redis.new
r.pipelined do 
  r.zrange('a', 'b', 5, :with_scores => true) 
end
```

This same code but with `with_scores` set to `false` raised this error instead:
```ruby
/Users/javierhonduco/.gem/ruby/2.4.0/gems/redis-4.0.1/lib/redis/pipeline.rb:123:in `value': ERR value is not an integer or out of range (Redis::CommandError)
```

### The problem
After some more combinations tested, the key part (no pun intended), was executing a failing command inside a pipeline that requires some parsing (aka transformations) being applied to them. When the transformation is called passing the result, as it's a `Redis::CommandError`[2], rather than an `Array`, [this](https://github.com/redis/redis-rb/blob/master/lib/redis.rb#L2810) raises the original exception. `FloatifyPairs` just checks that `array` is not `false`-y (`nil` or `false`).

### Proposed solution
Given that the problem is that we call `FloatifyPairs` passing it something that may, or may not be an `Enumerable`-like object, in this case, an `Array`, (and have `#each_slice` defined), this PR adds a check for that. In case `#each_slice ` is defined, it executes the current flow, otherwise, it just returns the argument verbatim, as this is going to be handled in https://github.com/redis/redis-rb/blob/master/lib/redis/pipeline.rb#L123, and the exception is going to be raised there. 

I chose this way of fixing the issue as its semantics are the same as the ones without the extra parsing required by `with_scores=true`.

Let me know if you would prefer to fix this somewhere else, happy to change this patch 😄 

#### Extra info

[1]:
We are using the one of the latest version of this gem (v4.0.1) and the testing has been done with master :)

Don't think that for this case is necessary, but for the sake of completeness: 
- Ruby version: 2.4.0 (MRI)
- Mac OS: 10.12.6

[2]: 
```ruby
[2805, 2814] in /Users/javierhonduco/.gem/ruby/2.3.3/gems/redis-4.0.1/lib/redis.rb
   2805:     }
   2806:
   2807:   FloatifyPairs =
   2808:     lambda { |array|
   2809:       debugger
=> 2810:       if array
   2811:         array.each_slice(2).map do |member, score|
   2812:           [member, Floatify.call(score)]
   2813:         end
   2814:       end
(byebug) array
#<Redis::CommandError: ERR value is not an integer or out of range>
(byebug) array.class
Redis::CommandError
```

Author: not-a-robot[bot]

lib/redis.rb

@@ -2885,11 +2885,13 @@ def method_missing(command, *args)
     }
 
   FloatifyPairs =
-    lambda { |array|
-      if array
-        array.each_slice(2).map do |member, score|
+    lambda { |result|
+      if result.respond_to?(:each_slice)
+        result.each_slice(2).map do |member, score|
           [member, Floatify.call(score)]
         end
+      else
+        result
       end
     }
 

test/pipelining_commands_test.rb

@@ -128,6 +128,14 @@ def test_futures_raise_when_trying_to_access_their_values_too_early
     end
   end
 
+  def test_futures_raise_when_command_errors_and_needs_transformation
+    assert_raise(Redis::CommandError) do
+      r.pipelined do
+        @result = r.zrange("a", "b", 5, :with_scores => true)
+      end
+    end
+  end
+
   def test_futures_can_be_identified
     r.pipelined do
       @result = r.sadd("foo", 1)