feat: add application/json support for client

twilio/http/http_client.py

@@ -84,7 +84,7 @@ def request(
             "hooks": self.request_hooks,
         }
 
-        if headers is not None and 'Content-Type' in headers.keys() and headers['Content-Type'] == 'application/json':
+        if headers and headers.get('Content-Type') == 'application/json':
             kwargs['json'] = data
         else:
             kwargs['data'] = data