Fix rekeying with GSS-API key exchange #1065
A mandatory feature of the SSH protocol does not work with GSS-API key exchange. Any attempt to renegotiate the transport keys for a gss-kex type transport causes a MIC failure and closes transport. Because ssh initiates a rekey operation after the transfer of 1 GB data, this bug can be a serious problem.
This pull should fix the bug. Also, there is a test case for rekeying with GSS-API created by @akruis.