http: Post base64 decoding tidy up

http: Post base64 decoding tidy up

Renamed copy_header_value() to Curl_copy_header_value() as this
function is now non static.

Simplified proxy flag in Curl_http_input_auth() when calling
sub-functions.

Removed unnecessary white space removal when using negotiate as it had
been missed in commit cdccb422671aeb.