Assets: don't add extension if other given and file exists

We should lookup if asset without appended extension exists.
When sprockets are disabled the asset tag helpers incorporate
this logic. When sprockets are enabled we should have the same
logic.

For example, we have style.ext file in app/assets/stylesheets and
we use stylesheet_link_tag in the layout. In this case we should
have /assets/style.ext instead of /assets/style.ext.css in the
output.

Closes #6310

actionpack/lib/sprockets/helpers/rails_helper.rb

@@ -155,8 +155,13 @@ module Sprockets
         end
 
         def rewrite_extension(source, dir, ext)
-          if ext && File.extname(source) != ".#{ext}"
-            "#{source}.#{ext}"
+          source_ext = File.extname(source)
+          if ext && source_ext != ".#{ext}"
+            if !source_ext.empty? && asset_environment[source]
+              source
+            else
+              "#{source}.#{ext}"
+            end
           else
             source
           end

actionpack/test/template/sprockets_helper_test.rb

@@ -307,6 +307,9 @@ class SprocketsHelperTest < ActionView::TestCase
     assert_match %r{\A<link href="/assets/style-[0-9a-f]+.css" media="screen" rel="stylesheet" type="text/css" />\Z},
       stylesheet_link_tag("style", "style")
 
+    assert_match %r{\A<link href="/assets/style-[0-9a-f]+.ext" media="screen" rel="stylesheet" type="text/css" />\Z},
+      stylesheet_link_tag("style.ext")
+
     @config.assets.compile = true
     @config.assets.debug = true
     assert_match %r{<link href="/stylesheets/application.css" media="screen" rel="stylesheet" type="text/css" />},