Queue: createMessage should throw for null message instead of submitting

[joostdenijs]

Dev estimate: 1
Test estimate: 1

Moved from private repository.

This code:
QueueContract service = createService();
service.createMessage("foo", null);
submits this XML to the server:

But the server doesn't like that:
InvalidXmlDocument: XML specified is not syntactically valid.
LineNumber: 0
LinePosition: 0
Would be nice to throw immediately instead of requiring the user to parse through the ServiceException themselves.

Similar issue for updateMessage when PopReciept is null: popreceiptquery parameter is required

[gcheng]

from @jcookems

The design we've been using for the APIs is to:

If bad data will cause the SDK code to error in other helper methods, which results in a confusinng client error (see #42), then validate those parameters.
If bad data causes the SDK to form a bad request, which results in a confusing server error (see #121), then validate those parameter.
Else, rely on the server to return a sensible error.

[jcookems]

Looks good. Unit tests are sufficient.