Autoserialization should work not only for dicts, but for lists too

[SergSlipushenko]

I hit this issue on httmock==1.2.3.

Traceback (most recent call last):
  File "/home/sslypushenko/projects/refstack/refstack/tests/unit/test_api.py", line 255, in test_get_capabilities
    result = self.controller.get()
  File "/home/sslypushenko/projects/refstack/refstack/api/controllers/v1.py", line 206, in get
    json = response.json()
  File "/home/sslypushenko/projects/refstack/.tox/py34/lib/python3.4/site-packages/requests/models.py", line 809, in json
    encoding = guess_json_utf(self.content)
  File "/home/sslypushenko/projects/refstack/.tox/py34/lib/python3.4/site-packages/requests/utils.py", line 631, in guess_json_utf
    nullcount = sample.count(_null)
TypeError: Can't convert 'bytes' object to str implicitly

I know that you fixed this issue for dicts, but content response can be a list too.
It is a kind of example for from our code:

        @httmock.all_requests
        def api_mock(url, request):
            headers = {'content-type': 'application/json'}
            content = [1,2,3]
            content = json.dumps(content)
            return httmock.response(200, content, headers, None, 5, request)