allow text/xml responses and do not allow unhandled content types

[nverwer]

According to RFC 7303, the response content type text/xml is allowed as an alias for application/xml. It took me a while to figure out why my model was not loaded, because nothing was shown when the content type is not recognized. I think Fore should throw an error in that case.

[JoernT]

thks at lot @nverwer