Fix asset_hash with content rule and uncommented CSS url

Fixes middleman/middleman#1852

middleman-core/features/asset_hash.feature

@@ -283,8 +283,15 @@ Feature: Assets get file hashes appended to them and references to them are upda
     When I cd to "build"
     Then the following files should exist:
       | javascripts/jquery.min-276c87ff.js |
+      | stylesheets/test-7de2ad06.css |
     And the following files should not exist:
       | javascripts/jquery.min.js |
+    And the file "stylesheets/test-7de2ad06.css" should contain:
+      """
+      .no-bug{background-image:url(/images/100px-5fd6fb90.jpg)}
+      .bug{content:"";background-image:url(/images/100px-5fd6fb90.jpg)}
+      .no-bug{content:""; background-image:url(/images/100px-5fd6fb90.jpg)}
+      """
 
   Scenario: Source map paths include the hash
     Given a successfully built app at "asset-hash-source-map"

middleman-core/fixtures/asset-hash-minified-app/source/stylesheets/test.css

@@ -0,0 +1,3 @@
+.no-bug{background-image:url(/images/100px.jpg)}
+.bug{content:"";background-image:url(/images/100px.jpg)}
+.no-bug{content:""; background-image:url(/images/100px.jpg)}

middleman-core/lib/middleman-core/util/rack.rb

@@ -22,7 +22,7 @@ module Middleman
 
     Contract String, String, ArrayOf[String], Proc => String
     def rewrite_paths(body, _path, exts, &_block)
-      matcher = /([\'\"\(,]\s*|# sourceMappingURL=)([^\s\'\"\)>]+(#{::Regexp.union(exts)}))/
+      matcher = /([\'\"\(,]\s*|# sourceMappingURL=)([^\s\'\"\)\(>]+(#{::Regexp.union(exts)}))/
 
       url_fn_prefix = 'url('