Fix another regression due to the inclusion of ActiveRecord::Error.

If a string is supplied to :default, it should not be used a
translation key (we already have :message for that).
[#3564 status:resolved]

Signed-off-by: Joshua Peek <josh@joshpeek.com>

activerecord/lib/active_record/validations.rb

@@ -158,7 +158,7 @@ def add_to_base(msg)
     # If +message+ is a Symbol, it will be translated, using the appropriate scope (see translate_error).
     #
     def add(attribute, message = nil, options = {})
-      options[:message] = options.delete(:default) if options.has_key?(:default)
+      options[:message] = options.delete(:default) if options[:default].is_a?(Symbol)
       error, message = message, nil if message.is_a?(Error)
 
       @errors[attribute.to_s] ||= []

activerecord/test/cases/validations_i18n_test.rb

@@ -521,6 +521,12 @@ def assert_full_message(message, *args)
     assert_equal message, ActiveRecord::Error.new(@reply, *args).full_message
   end
 
+  test ":default is only given to message if a symbol is supplied" do
+    store_translations(:errors => { :messages => { :"foo bar" => "You fooed: {{value}}." } })
+    @reply.errors.add(:title, :inexistent, :default => "foo bar")
+    assert_equal "foo bar", @reply.errors[:title]
+  end
+
   test "#generate_message passes the model attribute value for interpolation" do
     store_translations(:errors => { :messages => { :foo => "You fooed: {{value}}." } })
     @reply.title = "da title"