set content-type to json for beamline get routes

For beamline 'get attributes' routes, make sure Content-Type
header is 'application/json'.

The front-end is now checking the content-type header for replies,
and assumes an error unless it is set to 'application/json'.

mxcubeweb/routes/beamline.py

@@ -26,7 +26,7 @@ def get_func(name):
             **{"return": getattr(app.mxcubecore.get_adapter(name), attr)(**args)}
         )
 
-        return make_response(result.json(), 200)
+        return make_response(result.dict(), 200)
 
     route_url = f"{atype}/<string:name>/{attr}"
     endpoint = f"{atype}_{attr}"