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:. 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.