adding files with paths in the filename confuses maas

[Tim Penhey]

$ maas thumper files create filename=/som/path/test-file.txt file@=./.profile

$ maas thumper files read
**********************************************************************
*** WARNING! The API on the server differs from the description that
*** is cached locally. This may result in failed API calls. Refresh
*** the local API description with `maas refresh`.
**********************************************************************
Success.
Machine-readable output follows:
[
    {
        "resource_uri": "/MAAS/api/2.0/files//som/path/test-file.txt/",
        "anon_resource_uri": "/MAAS/api/2.0/files/?op=get_by_key&key=7bcfc714-fad3-11e5-932f-52540051bf22",
        "filename": "/som/path/test-file.txt"
    }
]

Note the double slash in the resource_uri