GSSAPI wrap/unwrap functions should honor max_buffer

[karth295]

If I call client.wrap(HUGE_BUFFER) or client.unwrap(HUGE_BUFFER), where HUGE_BUFFER > max_buffer, it should raise an exception. The caller should probably call wrap() or unwrap() in a loop based on client._chosen_mech.max_buffer.

[thobbs]

@karth295 thanks for the report! Does the commit above seem reasonable to you?

[karth295]

Is a buffer size negotiated in the plain algorithm? You only need to call wrap() or unwrap() if qop=auth-int or auth-conf, which it won't be for the plain mechanism.

Otherwise LGTM.