Autodetect encrypted data bag items in data_bag_item dsl method.

OK, ready to go @opscode/client-engineers. This resolves https://github.com/opscode/chef/issues/1849.


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Older messages
* :construction: Would like feedback on expected behavior @sersut @danielsdeleo @lamont-granquist :construction: 

It will be good to have a more beautiful error message here if decryption fails.