Fix generation of wrong json string when field has multiple errors

activemodel/lib/active_model/errors.rb

@@ -167,6 +167,16 @@ module ActiveModel
     def as_json(options=nil)
       self
     end
+    
+    def encode_json(encoder)
+      errors = []
+      each_pair do |key, value|
+        value = value.first if value.size == 1
+        errors << "#{encoder.encode(key.to_s)}:#{encoder.encode(value, false)}"
+      end
+
+      "{#{errors * ','}}"
+    end
 
     # Adds +message+ to the error messages on +attribute+, which will be returned on a call to
     # <tt>on(attribute)</tt> for the same attribute. More than one error can be added to the same

activemodel/test/cases/errors_test.rb

@@ -61,5 +61,15 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal ["name can not be blank", "name can not be nil"], person.errors.to_a
 
   end
+  
+  test 'to_json should return valid json string' do
+    person = Person.new
+    person.errors.add(:name, "can not be blank")
+    person.errors.add(:name, "can not be nil")
+    
+    hash = ActiveSupport::OrderedHash[:name, ["can not be blank", "can not be nil"]]
+    
+    assert_equal person.errors.to_json, hash.to_json
+  end
 
 end