Don't pass serializer option to associated serializers

Fixes #870

Commit af81a40 introduced passing a serializer's 'options'
along to its associated model serializers.

Thus, an explicit 'each_serializer' passed to render for a
singular resource would be passed on as the implicit 'serializer'
for its associations.

With @bf4

lib/active_model/serializer.rb

@@ -217,7 +217,7 @@ module ActiveModel
         if serializer_class
           serializer = serializer_class.new(
             association_value,
-            options.merge(serializer_from_options(association_options))
+            options.except(:serializer).merge(serializer_from_options(association_options))
           )
         elsif !association_value.nil? && !association_value.instance_of?(Object)
           association_options[:association_options][:virtual_value] = association_value