Ignore heartbeat messages based on content type.

https://www.pivotaltracker.com/story/show/82552398

Simply receiving them is enough for our monitoring to be able to detect inactive consumers, so we don't need to do any further processing.

In the tests I've used a fully formed document message where normally there would be a cut-down heartbeat message, because I want to prove that the code is ignoring the message based on the content type and not because it doesn't currently conform to spec (messages without topics are currently ignored).