Fix warning messages in "wait for external event" sample

[priyaananthasankar]

The RaiseEvent Function will throw a JSON decoding warning in red everytime when trying to decode its input, if I recall correctly. The warning looks like this:

[03/24/2020 00:24:06] Executed 'Functions.RaiseEvent' (Failed, Id=a93d2f7a-8e22-4948-a886-1bafd455db81)
[03/24/2020 00:24:06] System.Private.CoreLib: Exception while executing function: Functions.RaiseEvent. System.Private.CoreLib: Result: Failure
[03/24/2020 00:24:06] Exception: ContentTypeError: 0, message='Attempt to decode JSON with unexpected mimetype: ', url=URL('http://127.0.0.1:17071/durabletask/instances/ce2b661317a04af9ad8ffe45ecd59caf/raiseEvent/A')
[03/24/2020 00:24:06] Stack: File "/azure-functions-core-tools/workers/python/3.8/LINUX/X64/azure_functions_worker/dispatcher.py", line 312, in _handle__invocation_request
[03/24/2020 00:24:06] call_result = await fi.func(**args)
[03/24/2020 00:24:06] File "/code/azure-functions-durable-python/samples/external_events/RaiseEvent/init.py", line 34, in main
[03/24/2020 00:24:06] await client.raise_event(instance_id, event_name, True)
[03/24/2020 00:24:06] File "/usr/local/lib/python3.8/site-packages/azure/durable_functions/models/DurableOrchestrationClient.py", line 165, in raise_event
[03/24/2020 00:24:06] response = await self._post_async_request(request_url, json.dumps(event_data))
[03/24/2020 00:24:06] File "/usr/local/lib/python3.8/site-packages/azure/durable_functions/models/utils/http_utils.py", line 24, in post_async_request
[03/24/2020 00:24:06] data = await response.json()
[03/24/2020 00:24:06] File "/usr/local/lib/python3.8/site-packages/aiohttp/client_reqrep.py", line 1026, in json
[03/24/2020 00:24:06] raise ContentTypeError(
[03/24/2020 00:24:06]

[davidmrdavid]

This is fixed in: #85