botserver: Strip messages like we do in zulip-run-bot.

Previously, messages weren't stripped at all. This
caused most bots to break and send replies similar to
"I didn't understand your command". Nobody noticed,
because the tests were only validating that replies
were sent, but not the content in them. Thus, this
commit also adds tests to avoid further regressions.

zulip_botserver/tests/server_test_lib.py

@@ -35,6 +35,7 @@ class BotServerTestCase(TestCase):
             server.app.config["BOT_HANDLERS"] = bot_handlers
             server.app.config["MESSAGE_HANDLERS"] = message_handlers
 
+        mock_ExternalBotHandler.return_value.full_name = "test"
         response = self.app.post(data=json.dumps(event))
 
         # NOTE: Currently, assert_bot_server_response can only check the expected_response