Make ActiveModel::Serialization#read_attribute_for_serialization public

Despite the `#read_attribute_for_serialization` method being declared as
`private`, the module-level `ActiveModel::Serialization` documentation
mentions it explicitly by name.

Mentioning the method directly and encouraging classes to override it to
customize behavior breaks Rails existing `public`-`private` interface
convention.

This commit promotes the method to `public` visibility.

Author: seanpdoyle

activemodel/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Make `ActiveModel::Serialization#read_attribute_for_serialization` public
+
+    *Sean Doyle*
+
 *   Add a default token generator for password reset tokens when using `has_secure_password`.
 
     ```ruby

activemodel/lib/active_model/serialization.rb

@@ -29,8 +29,8 @@ module ActiveModel
   # An +attributes+ hash must be defined and should contain any attributes you
   # need to be serialized. Attributes must be strings, not symbols.
   # When called, serializable hash will use instance methods that match the name
-  # of the attributes hash's keys. In order to override this behavior, take a look
-  # at the private method +read_attribute_for_serialization+.
+  # of the attributes hash's keys. In order to override this behavior, override
+  # the +read_attribute_for_serialization+ method.
   #
   # ActiveModel::Serializers::JSON module automatically includes
   # the +ActiveModel::Serialization+ module, so there is no need to
@@ -148,29 +148,29 @@ def serializable_hash(options = nil)
       hash
     end
 
+    # Hook method defining how an attribute value should be retrieved for
+    # serialization. By default this is assumed to be an instance named after
+    # the attribute. Override this method in subclasses should you need to
+    # retrieve the value for a given attribute differently:
+    #
+    #   class MyClass
+    #     include ActiveModel::Serialization
+    #
+    #     def initialize(data = {})
+    #       @data = data
+    #     end
+    #
+    #     def read_attribute_for_serialization(key)
+    #       @data[key]
+    #     end
+    #   end
+    alias :read_attribute_for_serialization :send
+
     private
       def attribute_names_for_serialization
         attributes.keys
       end
 
-      # Hook method defining how an attribute value should be retrieved for
-      # serialization. By default this is assumed to be an instance named after
-      # the attribute. Override this method in subclasses should you need to
-      # retrieve the value for a given attribute differently:
-      #
-      #   class MyClass
-      #     include ActiveModel::Serialization
-      #
-      #     def initialize(data = {})
-      #       @data = data
-      #     end
-      #
-      #     def read_attribute_for_serialization(key)
-      #       @data[key]
-      #     end
-      #   end
-      alias :read_attribute_for_serialization :send
-
       def serializable_attributes(attribute_names)
         attribute_names.index_with { |n| read_attribute_for_serialization(n) }
       end