ConnectionExists: reusing possible HTTP+NTLM connections better

Make sure that the special NTLM magic we do is for HTTP+NTLM only since
that's where the authenticated connection is a weird non-standard
paradigm.

Regression brought in 8ae35102 (curl 7.35.0)

Bug: http://curl.haxx.se/mail/lib-2014-02/0100.html
Reported-by: Dan Fandrich