Fix a bug which caused creation of empty images (and volumes) to crash. FIxes #995.
- Runtime: add regression test for issue #995
Merge pull request #996 from dotcloud/995-volumes-crash

- Runtime: fix a bug which caused creation of empty images (and volumes) to crash.
Bump version to 0.4.6
Merge pull request #997 from dotcloud/bump_0.4.6

Bump version to 0.4.6