BLESession.handle_request: Fix startNotification KeyError

The value 'startNotification' is optional for the "read" request.
However, bluepy-scratch-link expects Scratch always set the value in the
requests. This may cause KeyError.

Signed-off-by: Shin'ichiro Kawasaki <kawasaki@juno.dti.ne.jp>

scratch_link.py

@@ -512,7 +512,7 @@ class BLESession(Session):
                 self.lock.release()
                 message = base64.standard_b64encode(b).decode('ascii')
                 res['result'] = { 'message': message, 'encode': 'base64' }
-            if params['startNotifications'] == True:
+            if params.get('startNotifications') == True:
                 logger.debug(f"start notification for {chara_id}")
                 service = self.perip.getServiceByUUID(UUID(service_id))
                 chas = service.getCharacteristics(forUUID=chara_id)