More errors in the append handler found by unit tests.  These have to do with
the passing of the mailbox name as a literal and what happens if the client
doesn't send all the data at once.
Add a quoted string test that passes the mailbox name in two parts.
I am slowly working my way down the coverage tests.  I noted that there was a
branch that was missed and it was associated with receiving the message data
in multiple chunks, so I set up a test for that, and found a couple of errors
associated with it, which I have now fixed.
Test sending the message data in multiple chunks
Make some changes because I discovered that AppendHandler::execute is only
called when m_parseStage is 2.
Add more tests.