Add with_content chain to have_json_path matcher

This PR adds a with_content() chain to the have_json_path matcher, which compares the content at the path with the one provided. I've found myself regularly in the position where not only the fact that the JSON path existed was important to me, but also the content. Of course you could split up this expectation up into two independent ones, but I think the with_content() makes the thing a lot easier.

I'm not totally happy with the failure messages - open for comments!