Calculates and returns a MIC (message integrity checksum) for
the given message that allows the receiver to verify its origin
and integrity through verify_mic or some other
GSS_VerifyMIC wrapper.
This wraps GSS_GetMIC according to RFC 2743 section 2.3.1.
This function requires that the context is established, or that
the early per-message protection service is available (c.f.
GSSAPI.PROT_READY_FLAG . If not, a
GSSAPI.MissingServicesError is thrown (but the context is not
closed).