SEAM/SPNEGO impl changed the delegated cred handling to return the cred for the
underlying mechanism directly - checks out bug#6290437 "gss_accept_sec_context needs better handling of delegated credentials" for more details.

Thus, the assumption made by native provider, e.g. the delegation cred has the same mech as the context, is no longer true when SEAM/SPNEGO fixed bug#6290437. Native provider should not assume the mech of delegated cred for SPNEGO and need to get the mech through native APIs.