Relationship fail to deserialize missing value

[kumy]

While upgrading our dependency from 0.16.0 to 0.20.0, we got a new stack trace

[…]
  File "/home/kumy/GIT/geokrety-api/venv/local/lib/python2.7/site-packages/marshmallow/schema.py", line 660, in _do_load
    index_errors=self.opts.index_errors,
  File "/home/kumy/GIT/geokrety-api/venv/local/lib/python2.7/site-packages/marshmallow/marshalling.py", line 295, in deserialize
    index=(index if index_errors else None)
  File "/home/kumy/GIT/geokrety-api/venv/local/lib/python2.7/site-packages/marshmallow/marshalling.py", line 68, in call_and_store
    value = getter_func(data)
  File "/home/kumy/GIT/geokrety-api/venv/local/lib/python2.7/site-packages/marshmallow/marshalling.py", line 288, in <lambda>
    data
  File "/home/kumy/GIT/geokrety-api/venv/local/lib/python2.7/site-packages/marshmallow_jsonapi/fields.py", line 208, in deserialize
    if 'links' in value:
TypeError: argument of type '_Missing' is not iterable

Posted data are:

{'data': {'relationships': {}, 'attributes': {'comment': 'My news comment'}, 'type': 'news-comment'}}

[kumy]

Please see PR #131