Fix rtm_send_message bug (#225)

Author: kt5356

slackclient/client.py

@@ -159,7 +159,7 @@ def rtm_send_message(self, channel, message, thread=None, reply_broadcast=None):
         # name and an attempt is made to find the ID in the workspace state cache.
         # If that lookup fails, the argument is used as the channel ID.
         found_channel = self.server.channels.find(channel)
-        channel_id = found_channel.id if found_channel.id else channel
+        channel_id = found_channel.id if found_channel else channel
         return self.server.rtm_send_message(
             channel_id,
             message,