Don't dup query values if they're frozen

We shouldn't dup query values if those values are frozen.  For example
if you have a script like:

```ruby
class Post < ActiveRecord::Base
  default_scope { where(author: "foo", title: "bar") }
end
```

The "foo" and "bar" strings would get dup'd even if you have
frozen_string_literals set.  I think this impacts all `where` calls, not
just default_scope, and I think frozen string literals are more common
so this should help more than just default_scope

Author: tenderlove

activerecord/lib/active_record/relation/query_attribute.rb

@@ -15,7 +15,9 @@ def initialize(...)
         elsif @type.serialized?
           value_for_database
         elsif @type.mutable? # If the type is simply mutable, we deep_dup it.
-          @value_before_type_cast = @value_before_type_cast.deep_dup
+          unless @value_before_type_cast.frozen?
+            @value_before_type_cast = @value_before_type_cast.deep_dup
+          end
         end
       end