Fix serializable_hash with xml generation and default :except option

When generating xml with a custom implementation of serializable_hash,
if using the :except option, it was being overriden by the default AR
implementation that attempts to ignore the inheritance column from STI
automatically. So, if you have an implementation like this:

    def serializable_hash(options={})
      super({ except: %w(some_attr) }.merge!(options))
    end

The :except option was correctly being used for :json generation, but
not for :xml, because the options hash already contained the :except
key with the inheritance column, thus overriding the customization.

This commit fixes this problem by removing the :except logic from the
xml serializer, that happened before calling serializable_hash. Since
serializable_hash also does the same check for inheritance column, this
logic was duplicated in both places, thus it's safe to remove it from
xml serializer (see ActiveRecord::Serialization#serializable_hash).

This is an attempt to solve issue #2498, that claims for a
"Single transformation API for both xml and json representations".

activerecord/lib/active_record/serializers/xml_serializer.rb

@@ -177,11 +177,6 @@ module ActiveRecord #:nodoc:
   end
 
   class XmlSerializer < ActiveModel::Serializers::Xml::Serializer #:nodoc:
-    def initialize(*args)
-      super
-      options[:except] = Array(options[:except]) | Array(@serializable.class.inheritance_column)
-    end
-
     class Attribute < ActiveModel::Serializers::Xml::Serializer::Attribute #:nodoc:
       def compute_type
         klass = @serializable.class

activerecord/test/cases/json_serialization_test.rb

@@ -115,6 +115,21 @@ class JsonSerializationTest < ActiveRecord::TestCase
     assert_no_match %r{ContactSti}, json
   end
 
+  def test_serializable_hash_with_default_except_option_and_excluding_inheritance_column_from_sti
+    @contact = ContactSti.new(@contact.attributes)
+    assert_equal 'ContactSti', @contact.type
+
+    def @contact.serializable_hash(options={})
+      super({ except: %w(age) }.merge!(options))
+    end
+
+    json = @contact.to_json
+    assert_match %r{"name":"Konata Izumi"}, json
+    assert_no_match %r{age}, json
+    assert_no_match %r{type}, json
+    assert_no_match %r{ContactSti}, json
+  end
+
   def test_serializable_hash_should_not_modify_options_in_argument
     options = { :only => :name }
     @contact.serializable_hash(options)

activerecord/test/cases/xml_serialization_test.rb

@@ -142,6 +142,21 @@ class DefaultXmlSerializationTest < ActiveRecord::TestCase
     assert_no_match %r{<type}, xml
     assert_no_match %r{ContactSti}, xml
   end
+
+  def test_serializable_hash_with_default_except_option_and_excluding_inheritance_column_from_sti
+    @contact = ContactSti.new(@contact.attributes)
+    assert_equal 'ContactSti', @contact.type
+
+    def @contact.serializable_hash(options={})
+      super({ except: %w(age) }.merge!(options))
+    end
+
+    xml = @contact.to_xml
+    assert_match %r{<name>aaron stack</name>}, xml
+    assert_no_match %r{age}, xml
+    assert_no_match %r{<type}, xml
+    assert_no_match %r{ContactSti}, xml
+  end
 end
 
 class DefaultXmlSerializationTimezoneTest < ActiveRecord::TestCase