Clean up MediaType config and add more fallback MediaTypes
This allows non-JSON endpoints to work without requesting the MediaType explicitly. It is necessary for the SAML SP endpoint which responds with XML.
Also cleaned up the related config:
- Removed obsolete, explicit Spring config. These values are the default for the current Spring Boot version.
- Replaced deprecated MediaType with UTF-8 charset. This MediaType was used in the past because of a Chrome bug which has already been fixed in 2017.
Fixes: #161 (closed)