Change content-type header

Author: TimPansino

newrelic/core/agent_protocol.py

@@ -579,7 +579,7 @@ def __init__(self, settings, host=None, client_cls=ApplicationModeClient):
                     k = k.encode("utf-8")
                 self._headers[k] = v
 
-        self._headers["Content-Type"] = "application/json"
+        self._headers["Content-Type"] = "application/x-protobuf"
         self._run_token = settings.agent_run_id
 
         # Logging