Creating a new HeaderHelper

This PR moves functions from the Util class to the HeaderHelper and deprecates the old functions.

It also introduces a new parser for the `Prefer:` header, which unfortunately is still incomplete, as [rfc7240](http://tools.ietf.org/html/rfc7240) has a number of bugs.
This was an edge case that happened because we used strtotime, which we do no longer.

There was no real use-case for it though. It was just to get to 100% code coverage.