Fix for broken asset_id when using relative_url_root

I noticed a bug in edge (v2.2.0) where asset_id timestamps were
missing from my rails app, this only happened for me when using
relative_url_root set to some value.

I've created a lighthouse ticket and patch attached to the ticket.
It's a very simple fix so hopefully it can make it into the build for
v2.2.1

Here's the ticket:
http://tinyurl.com/558um7

-- Joe

Applied. Thanks!