swift-simulator: correct handling of prefix

Before, a request with prefix=foo and an object named foo/bar.txt
would result in '/bar.txt' being returned.

This is incorrect since Swift really returns 'foo/' in that case. In
general, objects and subdirs are always returned using their full
name, so we should not cut off the prefix like we did.
swift-simulator: only add a subdir once in listObjects

Before, we would add {subdir: 'foo/'} twice when a container had two
objects named foo/a.txt and foo/b.txt.
Merge pull request #68 from mgeisler/trailing-subdir-slash

swift-simulator: correct handling of prefix