Add extension to middleman-sprockets to get http_prefix in Sass

We want to be able to prefix directory paths with the http_prefix in
Sass, however normally the `asset-path` helper tries to add a file
extension, so this commit adds a small extension that skips the usual
logic if the path ends with `/`.

Author: lfdebrux

lib/govuk_tech_docs.rb

@@ -22,6 +22,7 @@
 require "govuk_tech_docs/unique_identifier_generator"
 require "govuk_tech_docs/warning_text_extension"
 require "govuk_tech_docs/api_reference/api_reference_extension"
+require "govuk_tech_docs/http_prefix_extension"
 
 module GovukTechDocs
   # Configure the tech docs template
@@ -65,6 +66,7 @@ def self.configure(context, options = {})
     context.activate :unique_identifier
     context.activate :warning_text
     context.activate :api_reference
+    context.activate :sass_http_prefix
 
     context.helpers do
       include GovukTechDocs::TableOfContents::Helpers

lib/govuk_tech_docs/http_prefix_extension.rb

@@ -0,0 +1,22 @@
+module GovukTechDocs
+  class SassHttpPrefixExtension < Middleman::Extension
+    def after_configuration
+      app.extensions[:sprockets].environment.context_class.class_eval do
+        # Override the `asset-path()` helper available in Sprockets to
+        # return a directory rather than a file if the path ends with `/`
+        alias_method :orig_asset_path, :asset_path
+
+        def asset_path path, options={}
+          if options.empty? and path.end_with? "/"
+            File.join *[app.config[:http_prefix], path].compact
+          else
+            orig_asset_path path, options
+          end
+        end
+      end
+
+    end
+  end
+end
+
+::Middleman::Extensions.register(:sass_http_prefix, GovukTechDocs::SassHttpPrefixExtension)