dockerignore implementation assumes default Dockerfile name

[dnephin]

See docker/compose#1329 (comment) for the test case.

I believe the bug is here: https://github.com/docker/docker-py/blob/1.3.1/docker/client.py#L97

It should use dockerfile as the name instead of Dockerfile if it's not None.

[aanand]

I've worked a fix into #721.

[aanand]

Closed by #721.