Clarify that (clients and) services may include additional format parameters in Content-Type header.

Clients should be prepared for services to return custom format parameters, and should be able to safely ignore format parameters not specified in OData. Services must not include format parameters that a client needs to understand unless the client has done something special in order to request the format parameter. Services must return with not acceptable if the request contains an unknown or unsupported format parameter. Accepted: https://www.oasis-open.org/apps/org/workgroup/odata/download.php/52637/odata-meeting-69_on-20140327-minutes.html#odataprotocol