HTTP: CREDSPERREQUEST is for HTTP too

Commit 517b06d657ace (in 7.36.0) that brought the CREDSPERREQUEST flag
only set it for HTTPS, making HTTP less good at doing connection re-use
than it should be. Now set it for HTTP as well.

Simple test case

"curl -v -u foo:bar localhost --next -u bar:foo localhos"

Bug: http://curl.haxx.se/mail/lib-2014-05/0127.html
Reported-by: Kamil Dudka

lib/http.c

@@ -121,7 +121,7 @@ const struct Curl_handler Curl_handler_http = {
   ZERO_NULL,                            /* readwrite */
   PORT_HTTP,                            /* defport */
   CURLPROTO_HTTP,                       /* protocol */
-  PROTOPT_NONE                          /* flags */
+  PROTOPT_CREDSPERREQUEST               /* flags */
 };
 
 #ifdef USE_SSL