Add support for a default value

First, I fixed the spec so that both spec files will run when `rake spec` is executed.

Additionally, I updated the default value so that it is used for new records. Because overwriting the accessors to provide a default value isn't possible with serializers, it is necessary to handle it this way. I have updated the readme to show how these changes would be used (how we are using them here) and have written a spec for the behavior.