Fix using SCRIPT_NAME when not mounted on root
Fix using SCRIPT_NAME when not mounted on root

See #1394. This fixes Middleman assets when not mounted on root. Requires config[:http_prefix] to be set.
ryanmark/middleman-sprockets,middleman/middleman-sprockets,williamcoates/middleman-sprockets,middleman/middleman-sprockets,williamcoates/middleman-sprockets,ryanmark/middleman-sprockets,middleman/middleman-sprockets,williamcoates/middleman-sprockets,ryanmark/middleman-sprockets