Outputs headers in Pascal-Case.

I understand that according to the RFC headers aren't case-sensitive. But I ran into a server where they were, phantomjs-ghostdriver (version: 1.9.7). And because this library is used in @webdriverjs. Well you see the problem. The issue is fixed in the phantomjs webserver, but apparently still not shipped :shipit: or it's due to me running on windows (https://github.com/ariya/phantomjs/issues/11487). Since it doesn't really matter and most clients do output the headers in Pascal-Case. I thought it might be a good idea to address it here.

@camme I ran into the "cannot get a session token" issue. When connection directly to phantomjs from webdriverjs. With this "fix" it works.