return error if unsupported mimetype is requested

**Description**
why does pygeoapi return geojson if i send an accept header application/vnd.lala, expected is an error, status 406 requested mimetype not supported, exception could be if no accept header is sent, to return a json or html type encoding.

**Steps to Reproduce**
send header accept:application/vnd.lala to https://demo.pygeoapi.io/master/collections/dutch_castles/items?crs=http://www.opengis.net/def/crs/EPSG/0/28992

**Expected behavior**
error message `requested mimetype not supported`