implements plotly.Stream 'connected' property

- Stream.connected was always false before.  Now it
actually returns the state of the underlying connection

plotly/plotly/plotly.py

@@ -454,7 +454,6 @@ def __init__(self, stream_id):
 
         """
         self.stream_id = stream_id
-        self.connected = False
         self._stream = None
 
     def get_streaming_specs(self):
@@ -500,6 +499,13 @@ def heartbeat(self, reconnect_on=(200, '', 408)):
                 "cannot write to a closed connection. "
                 "Call `open()` on the stream to open the stream."
             )
+
+    @property
+    def connected(self):
+        if self._stream is None:
+            return False
+
+        return self._stream._isconnected()
 
     def open(self):
         """