Fix ls --recursive discrpency with prefixes

Fixes https://github.com/aws/aws-cli/issues/604

As an example of what is changed, start off with a bucket ``mybucket`` with the key ``foo/bar``.

When we used to run ``aws s3 ls s3://mybucket/foo/ --recursive``, it would list the object as ``foo/bar``. This is inconsistent with ``aws s3 ls s3://mybucket/foo/`` since it would list the object as ``bar``.

This change makes the output consistent with the non-recusive ``ls``.

cc @jamesls @danielgtaylor