Fix MSMQ label length

At one point we were sending messages whose definitions were nested deep down a long-winded namespace and started getting cryptic MSQM errors about invalid properties set on MSMQ messages. 

It turned out that even though [the spec](http://msdn.microsoft.com/en-us/library/ms703250(v=vs.85).aspx) allows up to 250 characters for the label (which is what `OutboundMsmqTransport` honoured), in reality it likes 249 a lot more. This is demonstrated by the test and fixed in the other commit.