Mark REST binary_sensor unavailable if request fails (#11506)

* Mark REST binary_sensor unavailable if request fails

* Add test suite for RESTful binary sensor

homeassistant/components/binary_sensor/rest.py

@@ -98,6 +98,11 @@ class RestBinarySensor(BinarySensorDevice):
         """Return the class of this sensor."""
         return self._device_class
 
+    @property
+    def available(self):
+        """Return the availability of this sensor."""
+        return self.rest.data is not None
+
     @property
     def is_on(self):
         """Return true if the binary sensor is on."""