Revert "Allow serializer_for to accept String instead of just class o…

…bjects"

Ref: 460d4c5

I first thought about making it configurable, but I just can't find a
justification for this feature in a very stable gem with such backward
compatibility impact.

lib/active_model/serializer.rb

@@ -133,11 +133,7 @@ def build_serializer_class(resource, options)
         klass_name = +""
         klass_name << "#{options[:namespace]}::" if options[:namespace]
         klass_name << options[:prefix].to_s.classify if options[:prefix]
-        if resource.is_a?(String)
-          klass_name << "#{resource}Serializer"
-        else
-          klass_name << "#{resource.class.name}Serializer"
-        end
+        klass_name << "#{resource.class.name}Serializer"
       end
 
       def associate(klass, *attrs)

test/unit/active_model/array_serializer/serialization_test.rb

@@ -49,7 +49,7 @@ def test_serializer_for_array_returns_appropriate_type
         def object.serializer_class; CustomSerializer; end
 
         assert_equal CustomSerializer, Serializer.serializer_for(object)
-        assert_equal CustomSerializer, Serializer.serializer_for('Custom')
+        assert_nil Serializer.serializer_for('Custom')
       end
     end