! httpx: enable FEOU and FSOD to be interchanged in the usual cases, fixes #426

The idea is to
  - interpret application/x-www-form-urlencoded fields as text/plain so that FEOU can read
    it as well
  - interpret multipart form parts with content-type text/plain as strings that
    can be read from FSOD

This should make it much easier to change the form encoding on the client
without having to change anything on the spray side.