Always put a slash between the checksum and path in filebucket URLs

Since absolute paths on Windows do not always start with /, we need to
make sure that there is always a slash between the checksum and the
path, or the drive letter will end up being considered as part of the
checksum.

On systems where absolute paths always start with /, the extra slash
is removed by the parsing done to the constructed URL.

Reviewed-by: Nick Lewis <nick@puppetlabs.com>
(cherry picked from commit 5314376d4378c4b4f990a7d61a9677594e12a2a5)