Respond to Alexa scene activation correctly

The API documentation[1] specifies that Alexa.SceneController Activate
must get a ActivationStarted response. Responding with just a `Response`
will elicit a "Hmm... $scene is not responding" from Alexa.

[1]: https://developer.amazon.com/docs/smarthome/provide-scenes-in-a-smart-home-skill.html

homeassistant/components/alexa/smart_home.py

@@ -2,6 +2,7 @@
 import asyncio
 import logging
 import math
+from datetime import datetime
 from uuid import uuid4
 
 from homeassistant.components import (
@@ -70,6 +71,39 @@
 }
 
 
+class _Cause(object):
+    """Possible causes for property changes.
+
+    https://developer.amazon.com/docs/smarthome/state-reporting-for-a-smart-home-skill.html#cause-object
+    """
+
+    # Indicates that the event was caused by a customer interaction with an
+    # application. For example, a customer switches on a light, or locks a door
+    # using the Alexa app or an app provided by a device vendor.
+    APP_INTERACTION = 'APP_INTERACTION'
+
+    # Indicates that the event was caused by a physical interaction with an
+    # endpoint. For example manually switching on a light or manually locking a
+    # door lock
+    PHYSICAL_INTERACTION = 'PHYSICAL_INTERACTION'
+
+    # Indicates that the event was caused by the periodic poll of an appliance,
+    # which found a change in value. For example, you might poll a temperature
+    # sensor every hour, and send the updated temperature to Alexa.
+    PERIODIC_POLL = 'PERIODIC_POLL'
+
+    # Indicates that the event was caused by the application of a device rule.
+    # For example, a customer configures a rule to switch on a light if a
+    # motion sensor detects motion. In this case, Alexa receives an event from
+    # the motion sensor, and another event from the light to indicate that its
+    # state change was caused by the rule.
+    RULE_TRIGGER = 'RULE_TRIGGER'
+
+    # Indicates that the event was caused by a voice interaction with Alexa.
+    # For example a user speaking to their Echo device.
+    VOICE_INTERACTION = 'VOICE_INTERACTION'
+
+
 class Config:
     """Hold the configuration for Alexa."""
 
@@ -400,7 +434,17 @@ def async_api_activate(hass, config, request, entity):
         ATTR_ENTITY_ID: entity.entity_id
     }, blocking=False)
 
-    return api_message(request)
+    payload = {
+        'cause': {'type': _Cause.VOICE_INTERACTION},
+        'timestamp': '%sZ' % (datetime.utcnow().isoformat(),)
+    }
+
+    return api_message(
+        request,
+        name='ActivationStarted',
+        namespace='Alexa.SceneController',
+        payload=payload,
+    )
 
 
 @HANDLERS.register(('Alexa.PercentageController', 'SetPercentage'))

tests/components/alexa/test_smart_home.py

@@ -739,7 +739,9 @@ def test_api_activate(hass, domain):
 
     assert len(call) == 1
     assert call[0].data['entity_id'] == '{}.test'.format(domain)
-    assert msg['header']['name'] == 'Response'
+    assert msg['header']['name'] == 'ActivationStarted'
+    assert msg['payload']['cause']['type'] == 'VOICE_INTERACTION'
+    assert 'timestamp' in msg['payload']
 
 
 @asyncio.coroutine