Coverage improvement by adding test code, etc.

[nsano-rururu]

Remove unnecessary code

dingtalk.py

I left behind unnecessary code. I'm sorry.

    def format_body(self, body):
        return body.encode('utf8')

opsgeine.py

I found that the code removed in Yelp/elastalert#1876 on the yelp / elastalert pull request was accidentally revived and left in the Yelp/elastalert#1935 pull request, so I removed it.

    def _fill_responders(self, responders, type_):
        return [{'id': r, 'type': type_} for r in responders]

Improved test coverage

TOTAL
68% → 70%

dingtalk.py
97% → 100%
email.py
89% → 90%
exotel.py
61% → 100%
jira.py
74% → 76%
opsgeine.py
92% → 96%
ses.py
37% → 62%
sns.py
53% → 83%
stomp.py
20% → 81%
twilio.py
83% → 100%
zabbix.py
54% → 57%

[jertel]

Thanks again for continuing to raise the unit test coverage!