Added an MSMQ label length test

MSMQ docs mention a maximum of 250 characters for a message label and
the code takes that into account. Yet when a message label is set to
exactly 250 characters, MSMQ does not like it very much and throws a
MessageQueueException: "One or more of the passed properties are
invalid."
Adjust MSMQ message label length

...to just under the maximum supposedly allowed to make it happy.
Merge pull request #235 from davidcie/bugfix-msmq-label

Fix MSMQ label length