Add request headers manually for redirects.

We need to specify the Accept header for the Capabilities request.
See:
[Issue 4476 - selenium - 500 Error, when using `/session/:sessionId` to retrieve the capabilities of the specified session.](https://code.google.com/p/selenium/issues/detail?id=4476#c7)

http.Client now follows POST redirects, but it does not copy request headers. So we need to do it ourselves for now.
See:
[Issue 4800 - go - net/http: copy headers when following redirects](https://code.google.com/p/go/issues/detail?id=4800)