Report correct thermostat mode to Alexa (#18053)

We were erroneously reporting the _previous_ mode. So if the thermostat was off
and the user asks, "Alexa, set the thermostat to heat", the thermostat would be
set to heat but Alexa would respond, "The thermostat is off."

Bug caught by @Thunderbird2086 at
https://github.com/home-assistant/home-assistant/pull/17969#issuecomment-434654345

homeassistant/components/alexa/smart_home.py

@@ -1839,11 +1839,17 @@ async def async_api_set_thermostat_mode(hass, config, directive, context):
         climate.ATTR_OPERATION_MODE: ha_mode,
     }
 
+    response = directive.response()
     await hass.services.async_call(
         entity.domain, climate.SERVICE_SET_OPERATION_MODE, data,
         blocking=False, context=context)
+    response.add_context_property({
+        'name': 'thermostatMode',
+        'namespace': 'Alexa.ThermostatController',
+        'value': mode,
+    })
 
-    return directive.response()
+    return response
 
 
 @HANDLERS.register(('Alexa', 'ReportState'))