Fix #4319 - source file url sent to the master is invalid

We were sending an incorrect (containing a //) url for sourced file
content since the file streaming patches.
Depending on the webserver in front of puppet it could fail (for instance
nginx+mongrel).

This patch fixes the offending // in each sourced file urls.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>