OAuth1Client: Allow to set a content type

This allows to set a content type for POST (and other) requests in the OAuth1 client. This is especially useful to communicate the type of the request body (see #236), but some APIs also require an explicit `application/x-www-form-urlencoded`.

If #236 is not accepted, I can also decouple this from it.

(In my case I need at least one of those changes to post to the [XING API](https://dev.xing.com/docs))