http: don't give up on auth on the first try

When the server rejects an authentication request, ask the caller for
the credentials again, instead of giving up on the first try.

src/transports/http.c

@@ -336,8 +336,7 @@ static int on_headers_complete(http_parser *parser)
 		if (!t->owner->cred_acquire_cb) {
 			no_callback = 1;
 		} else {
-			if (allowed_auth_types &&
-			    (!t->cred || 0 == (t->cred->credtype & allowed_auth_types))) {
+			if (allowed_auth_types) {
 
 				error = t->owner->cred_acquire_cb(&t->cred,
 								  t->owner->url,