Fix rate limiting when 'Retry-After' is missing

[slemiere]

Hello,

I was having the following issue when using this library.

  File "/usr/lib/python3.6/site-packages/nest/nest.py", line 578, in target
    self._set('devices/thermostats', data)
  File "/usr/lib/python3.6/site-packages/nest/nest.py", line 206, in _set
    response = self._nest_api._put(path=path, data=data)
  File "/usr/lib/python3.6/site-packages/nest/nest.py", line 1865, in _put
    return self._request('PUT', path, data=data)
  File "/usr/lib/python3.6/site-packages/nest/nest.py", line 1853, in _request
    default_wait=5)
  File "/usr/lib/python3.6/site-packages/nest/nest.py", line 1696, in _handle_ratelimit
    retry_after = response.headers['Retry-After']
  File "/usr/lib/python3.6/site-packages/requests/structures.py", line 52, in __getitem__
    return self._store[key.lower()][1]
KeyError: 'retry-after'

Looking into it it seems that Nest is not always sending back a retry-after header. I believe using get instead of the [] accessor should solve this issue.

[balloob]

Looks good.