Bugfix cloud webhooks text response

[pvizeli]

Description:

I change it to text because we only support text as response and aioHttp perform the correct charset that not need to be utf-8 like we set now static.

Fix:

Error handling message
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/homeassistant/components/cloud/iot.py", line 237, in _handle_connection
    hass, self.cloud, msg['handler'], msg['payload'])
  File "/usr/local/lib/python3.6/site-packages/homeassistant/components/cloud/iot.py", line 293, in async_handle_message
    return (yield from handler(hass, cloud, payload))
  File "/usr/local/lib/python3.6/site-packages/homeassistant/components/cloud/iot.py", line 366, in async_handle_webhook
    body = body.decode('utf-8')
AttributeError: 'StringPayload' object has no attribute 'decode'

[balloob]

Since this is a generic helper, we should not change its behavior to make it more shaped for one implementation. If we want that, we should move this function inside the cloud integration.