Allow setting executor for zipped promises

The use case for this is a little strange, since you can set the
executors for the zipped promises when they're created instead. In my
case, it would make it easier to run a number of immediately executed
promises on a pool:

```
p1 = Concurrent::Promise.new(executor: :immediate) { ... }
p2 = Concurrent::Promise.new(executor: :immediate) { ... }

...

Concurrent::Promise.zip(p1, p2, executor: SOME_THREAD_POOL).then { ... }
```

Author: davishmcclurg

lib/concurrent/promise.rb

@@ -387,7 +387,9 @@ def flat_map(&block)
     #
     # @return [Promise<Array>]
     def self.zip(*promises)
-      zero = Promise.new(executor: ImmediateExecutor.new) { [] }
+      opts = promises.last.is_a?(::Hash) ? promises.pop : {}
+      opts[:executor] ||= ImmediateExecutor.new
+      zero = Promise.new(opts) { [] }
 
       promises.reduce(zero) do |p1, p2|
         p1.flat_map do |results|

spec/concurrent/promise_spec.rb

@@ -363,6 +363,14 @@ def get_ivar_from_args(opts)
         expect(composite).to be_unscheduled
       end
 
+      it 'allows setting executor for Promise chain' do
+        new_executor = Concurrent::SingleThreadExecutor.new
+        promise = promise1.zip(promise2, promise3, executor: new_executor)
+
+        promise = promise.instance_variable_get(:@parent) until promise.send(:root?)
+        expect(promise.instance_variable_get(:@executor)).to be(new_executor)
+      end
+
       it 'yields the results as an array' do
         composite = promise1.zip(promise2, promise3).execute.wait
 
@@ -387,6 +395,14 @@ def get_ivar_from_args(opts)
         expect(composite).to be_unscheduled
       end
 
+      it 'allows setting executor for Promise chain' do
+        new_executor = Concurrent::SingleThreadExecutor.new
+        promise = Promise.zip(promise1, promise2, promise3, executor: new_executor)
+
+        promise = promise.instance_variable_get(:@parent) until promise.send(:root?)
+        expect(promise.instance_variable_get(:@executor)).to be(new_executor)
+      end
+
       it 'yields the results as an array' do
         composite = Promise.zip(promise1, promise2, promise3).execute.wait