Infinite loop for null message body

Stomp._transmit calls body.length, which throws an exception when the body is nil. The exception is caught inside transmit, which calls _transmit again from w/in a rescue block. This creates a lot of sockets and hammers the shit out of the broker (I get OutOfMemoryError in activeMQ). I feel 0 is good for nil messages, let me know if you think the message shouldn't be sent and I'll change the patch or write a test.