Fix bug with :suppress_content_length on STOMP 1.1 protocol

When using STOMP 1.1 the header encoding done breaks the test for :suppress_content_length header option - preventing the gem from ever being able to send a TextMessage style message (instead always sending a ByteMessage type).

The following patch changes the STOMP header encoding method so that the keys are symbols instead of strings. This doesn't affect the message submission (as they're all converted to strings to be sent) but it allows the code in `_transmit` to correctly detect the :suppress_content_length header and do just that.

Also, the example in the documentation had be confused for a long time - obviously #send is a reserved method name, so now the library is using #publish, but the README.rdoc hadn't been updated. This little patch fixes that!