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.
Allow customizing context with requests

There are times when the ability to pass custom headers comes in handy,
but this was only possible when using .Publish and not .PublishRequest
or .SendRequest.
Adding tests covering customizing context

Two simple tests demonstrating that the added methods are called and are
able to e.g. inject values into message headers.
Merge pull request #235 from davidcie/bugfix-msmq-label

Fix MSMQ label length
Merge pull request #236 from davidcie/feature-publish-customizations

Allow customizing {Publish,Send}Request context