Correctly handle unicode data in the HTTP runner

[Kami]

HTTP runner didn't correctly handle data argument when it was an unicode string. When providing a textual (non-object / dictionary) data, requests expects bytes and not unicode type and that's what we should pass to the function to make it work correctly.

Before:

st2 run core.http url="http://httpbin.org/post" headers='{"Content-Type": "application/json; charset=utf-8"}' method=POST body='{ "msg": "ažž" }'
id: 5c8bafe5076129241e7452a2
status: failed
parameters: 
  body: "{ "msg": "ažž" }"
  headers:
    Content-Type: application/json; charset=utf-8
  method: POST
  url: http://httpbin.org/post
result: 
  error: '''ascii'' codec can''t encode characters in position 11-12: ordinal not in range(128)'
  traceback: "  File "/data/stanley/st2actions/st2actions/container/base.py", line 127, in _do_run
    (status, result, context) = runner.run(action_params)
  File "/data/stanley/contrib/runners/http_runner/http_runner/http_runner.py", line 102, in run
    result = client.run()
  File "/data/stanley/contrib/runners/http_runner/http_runner/http_runner.py", line 226, in run
    verify=self.verify
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/requests/api.py", line 60, in request
    return session.request(method=method, url=url, **kwargs)
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/requests/sessions.py", line 533, in request
    resp = self.send(prep, **send_kwargs)
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/requests/sessions.py", line 646, in send
    r = adapter.send(request, **kwargs)
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/requests/adapters.py", line 449, in send
    timeout=timeout
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/urllib3-1.24.1-py2.7.egg/urllib3/connectionpool.py", line 600, in urlopen
    chunked=chunked)
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/urllib3-1.24.1-py2.7.egg/urllib3/connectionpool.py", line 354, in _make_request
    conn.request(method, url, **httplib_request_kw)
  File "/usr/lib/python2.7/httplib.py", line 1057, in request
    self._send_request(method, url, body, headers)
  File "/usr/lib/python2.7/httplib.py", line 1097, in _send_request
    self.endheaders(body)
  File "/usr/lib/python2.7/httplib.py", line 1053, in endheaders
    self._send_output(message_body)
  File "/usr/lib/python2.7/httplib.py", line 901, in _send_output
    self.send(message_body)
  File "/usr/lib/python2.7/httplib.py", line 873, in send
    self.sock.sendall(data)
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/eventlet/greenio/base.py", line 401, in sendall
    tail = self.send(data, flags)
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/eventlet/greenio/base.py", line 395, in send
    return self._send_loop(self.fd.send, data, flags)
  File "/data/stanley/virtualenv/local/lib/python2.7/site-packages/eventlet/greenio/base.py", line 382, in _send_loop
    return send_method(data, *args)
"

After:

st2 run core.http url="http://httpbin.org/post" headers='{"Content-Type": "application/json; charset=utf-8"}' method=POST body='{ "msg": "ažž" }'

id: 5c8bb37007612924fa8c92cd
status: succeeded
parameters: 
  body: "{ "msg": "ažž" }"
  headers:
    Content-Type: application/json; charset=utf-8
  method: POST
  url: http://httpbin.org/post
result: 
  body:
    args: {}
    data: "{ "msg": "ažž" }"
    files: {}
    form: {}
    headers:
      Accept: '*/*'
      Accept-Encoding: gzip, deflate
      Content-Length: '18'
      Content-Type: application/json; charset=utf-8
      Host: httpbin.org
      User-Agent: st2/v3.0dev
      X-Stanley-Action: http
    json:
      msg: "ažž"
    url: https://httpbin.org/post
  headers:
    Access-Control-Allow-Credentials: 'true'
    Access-Control-Allow-Origin: '*'
    Connection: keep-alive
    Content-Encoding: gzip
    Content-Length: '288'
    Content-Type: application/json
    Date: Fri, 15 Mar 2019 14:17:40 GMT
    Server: nginx
  parsed: true
  status_code: 200

I confirmed the fix under Python 2.7 and Python 3.6.

Resolves #4599.

Thanks to @kknyxkk and @rsmartins78 for reporting this.

TODO

• Tests
• Changelog entry