bots: Add complete tests for offline testing of yoda bot.

Since yoda api returns response of text form, made changes
in test_lib.py to handle responses that can either be json
or plain text.

zulip_bots/zulip_bots/bots/yoda/fixtures/test_1.json

@@ -0,0 +1,14 @@
+{
+  "request": {
+    "api_url": "https://yoda.p.mashape.com/yoda?sentence=You+will+learn+how+to+speak+like+me+someday.",
+    "headers": {
+      "X-Mashape-Key": "12345678",
+      "Accept": "text/plain"
+    }
+  },
+  "response": {"text": "Learn how to speak like me someday, you will. Yes, hmmm."},
+  "response-headers": {
+    "status": 200,
+    "content-type": "text/html; charset=utf-8"
+  }
+}

zulip_bots/zulip_bots/bots/yoda/test_yoda.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python
+
+from __future__ import absolute_import
+from __future__ import print_function
+
+import json
+
+from zulip_bots.test_lib import BotTestCase
+
+class TestYodaBot(BotTestCase):
+    bot_name = "yoda"
+
+    def test_bot(self):
+        bot_response = "Learn how to speak like me someday, you will. Yes, hmmm."
+
+        with self.mock_config_info({'api_key': '12345678'}), \
+                self.mock_http_conversation('test_1'):
+            self.initialize_bot()
+            self.assert_bot_response(
+                message = {'content': 'You will learn how to speak like me someday.'},
+                response = {'content': bot_response},
+                expected_method='send_reply'
+            )

zulip_bots/zulip_bots/bots/yoda/yoda.py

@@ -58,7 +58,6 @@ class YodaSpeakHandler(object):
 
     def send_to_yoda_api(self, sentence):
         # function for sending sentence to api
-
         response = requests.get("https://yoda.p.mashape.com/yoda?sentence=" + sentence,
                                 headers={
                                     "X-Mashape-Key": self.api_key,