Fixed a bug in write_entry()

[nengxu]

Thanks for creating this gem. I encountered a problem when using it:

  Rails.cache.write(key, trans)
  trans.delete_if {|key, value| value[:cen].blank? }

will raise an error saying trans is frozen, where trans is a hash. However, the above statements worked fine with file store.

I forked and fixed this bug as in:
http://github.com/nengxu/mongo_store/commit/f72c013e33fc387058968381bc42b2a0a738688b

Please review. Thanks.