Extend response headers to response

[nmuesch]

Properly extend the response headers to the response object list.

Resolves:

Traceback (most recent call last):
  File "test.py", line 14, in <module>
    print (api.Monitor.get_all())
  File "/usr/local/lib/python3.7/site-packages/datadog/api/monitors.py", line 57, in get_all
    return super(Monitor, cls).get_all(**params)
  File "/usr/local/lib/python3.7/site-packages/datadog/api/resources.py", line 181, in get_all
    return APIClient.submit('GET', cls._resource_name, api_version, **params)
  File "/usr/local/lib/python3.7/site-packages/datadog/api/api_client.py", line 161, in submit
    response_obj['response_headers'] = response_headers
TypeError: list indices must be integers or slices, not str

issues on endpoints that had response headers returned.

Fixes #393

[aamangeldi]

@ofek @nmuesch this seems to have broken sth on my end, which works fine in 0.29.0:

/databricks/python/lib/python3.5/site-packages/datadog/api/metrics.py in send(cls, metrics, attach_host_name, **single_metric)
     89             raise KeyError("'points' parameter is required")
     90 
---> 91         return super(Metric, cls).send(attach_host_name=attach_host_name, **metrics_dict)
     92 
     93     @classmethod

/databricks/python/lib/python3.5/site-packages/datadog/api/resources.py in send(cls, attach_host_name, id, **body)
     76         if id is None:
     77             return APIClient.submit('POST', cls._resource_name, api_version, body,
---> 78                                     attach_host_name=attach_host_name)
     79 
     80         path = '{resource_name}/{resource_id}'.format(

/databricks/python/lib/python3.5/site-packages/datadog/api/api_client.py in submit(cls, method, path, api_version, body, attach_host_name, response_formatter, error_formatter, **params)
    160             if response_obj:
    161                 for response in response_obj:
--> 162                     response['response_headers'] = response_headers
    163 
    164             if response_formatter is None:

TypeError: 'str' object does not support item assignment