In rails4, when config.asset_host is defined as a proc, returning nil from that proc no longer behaves the same as if config.asset_host were not configured, but rather treats asset_host as an empty string, which is not useful, because it gives urls like this:
This PR restores the previous behavior (and adds test coverage).
restore ability to return nil from asset_host proc, add test
Fix empty host for an asset url when ActionController::Base.asset_hos…
…t returns a proc that evaluates to nil