Dirty attributes methods don't work.

[bdurand]

The ActiveModel dirty attribute methods (i.e. foo_changed?, foo_was, foo_change, reset_foo!) don't work. ActiveModel::Dirty stores changes in a Hash with String keys, but MongoModel::AttributeMethods::Dirty#write_attribute adds the changes with symbol attributes.

A fix with tests can be found here:

http://github.com/bdurand/mongomodel/commit/8ed71f994669c8ba1c7f8f8eccd1c84e3de530dd