Fix https proxy authentication, the header was missing a colon so that

the auth info was not working.
The fix for the missing header that was still present in 2013-12-08 "Binky" was missing the colon after the header keyword.
I have written a unit test to verify this locally, however it makes not much sense to include the test in the normal source since that requires a proxy-auth running on the test proxy (if that can be set up, adding the user/pw to the unit tests httpclient_http*_proxy.test.php would be possible)