Added #decorated? method to Draper::CollectionDecorator

Closes #447
Merge pull request #460 from carloslopes/decorated-collection

Add #decorated? method to CollectionDecorator
Extract Factory from DecoratedAssociation
Fix minor decorated association context bug

If the context of the owner decorator changes, the default context
of the decorated association should change too.
Add decorates_assigned method to controllers
Merge pull request #461 from haines/decorates_assigned

Add decorates_assigned method to controllers
Prevent deprecation warning in rails console
Add respond_to_missing?

Causes #method to return delegated methods on decorators in Ruby 1.9.2+
Remove Ruby version guards on respond_to_missing?
Merge pull request #466 from mattkimmel/respond-to-missing

Add respond_to_missing?
Uncomment accidentally-commented line :blush:
Support STI in Decoratable.decorate

Closes #468
Merge pull request #470 from haines/sti

Support STI in Decoratable.decorate
Allow serialization of CollectionDecorators with ActiveModel::Serializers

Closes #471
Merge pull request #473 from haines/am_serializers

Allow serialization of CollectionDecorators with ActiveModel::Serializers