CAS-1166/1168 (3.5.x) login form is not shown on error of Spnego/login form is not shown on error of Spnego

This fix will essentially revert CAS-1065, but allow implementers to optionally invoke it. Without this fix SPNEGO will not display the CAS login form for clients that aren't setup to participate in SPNEGO handshaking.

(A CAS 4.X version will be submitted shortly.)