api/server: fallback to application/json as best mimetype for content nego

we almost never return text/plain or text/html, so json is a better default value.

---

Migrated from D1256 (view on Phabricator)

swh/core/api/__init__.py

@@ -30,7 +30,7 @@ logger = logging.getLogger(__name__)
 class Negotiator(NegotiatorBase):
     def best_mimetype(self):
         return request.accept_mimetypes.best_match(
-            self.accept_mimetypes, 'text/html')
+            self.accept_mimetypes, 'application/json')
 
     def _abort(self, status_code, err=None):
         return abort(status_code, err)