HTTP: CREDSPERREQUEST is for HTTP too

Commit 517b06d6 (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