Fixes error when asset_host is not a string.

[tscolari]

Rails' asset_host can be either a string or an object that respond to call (e.g. Proc). There was an error because when it was a Proc it was trying to concatenate it as a string.

Since at assets:precompile we don't know about the request to pass it forward to the Proc/Class, the better we can do is probably use the relative path.

[spohlenz]

Great work. Thank you.

[tscolari]

:) Thank you!