CHEF-3975: Fixed data bag item indexing

Data bag indexing was not implemented correctly. The Ruby server sends
the data bag with the original data. Erchef had been nesting the data
bag data under the key, "raw_data". This causes knife search to fail.

Specifically, the Ruby code calls #to_hash, not #to_json, and the Erlang
code had implemented #to_json.

For reference:

https://github.com/opscode/chef/blob/10-stable/chef/lib/chef/data_bag_item.rb#L137-L143