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'
Not to be pedantic, but [`#key?` is preferred to `#has_key?`](https://github.com/bbatsov/ruby-style-guide#hash-key), the latter is considered to be deprecated.