Since upstream commit a34d5ef53c2e (sshpeer: move logic for sending a
request into a new function, 2018-02-21), peer._call() and
peer._callcompressible() no longer call into
peer._callstream(). Instead, all three call into a new
peer._sendrequest(). We therefore need to override that new method.
The _sendrequest() method will gain another argument ("framed") in a
later commit, so this patch also allows extra arguments to be passed
to it.