Fix to 400 Returns on spreadsheet.document_feed_entry

As of Feb 14, a 400 is returned on spreadsheet.document_feed_entry with an otherwise active and working client (i.e. spreadsheet row/col returns are successful). I had been using the gem since July with no issues prior. 

Further reports from other users: http://stackoverflow.com/questions/21778342/response-code-400-for-get-https-docs-google-com-feeds-documents-private-full & https://github.com/gimite/google-drive-ruby/issues/76

This is fixed by both appending GData Version and updating document_feed_url.