Fix Docker::Image.create when `m` is nil

Sometime when pulling for second time the API responds
with nil somewhere, leading to:

       undefined method `[]' for nil:NilClass
     # /var/lib/gems/2.1.0/gems/docker-api-1.13.7/lib/docker/image.rb:102:in `create'
I think it'd be much clearer if we didn't have the reverse, find, has_key, and fetch.  The select above is pretty clear. Can you change it to that?