Enabled the dumping of the parsing result

Hi,


I've just came up with a solution for something that came up. On our project we use this gem to parse something that will be cached. The caching calls the marshal dump on the value, and it somehow wasn't working because the result of the Crack::XML.parse was not deemed dumpable. I found out this was due to some dynamic insertion of methods, which makes this object not dumpable anymore. So I corrected it. Hope you find the solution good. 

Kind Regards,
Tiago