Add gm raw support

https://developers.google.com/gmail/imap_extensions?csw=1#extension_of_the_search_command_x-gm-raw

gm raw has been on the [todo list](https://github.com/nu7hatch/gmail/commit/687bb4077a3b1a3caacdd7c069df01582e651d18) since 2012. [ruby-gmail](https://github.com/dcparker/ruby-gmail/blob/f8c2651f19cb41d9efb9029e01b281a609ffe32c/lib/gmail/mailbox.rb#L66) has supported it for a bit. It's a one line fix. I've added this feature to the gmail gem.