Provide image_path that works with Sprockets

Prior to this, functions in sprockets-sass conflicted with ours and caused an infinite loop and StackError. This fixes thomas-mcdonald/bootstrap-sass#257

Note that Sprockets' asset_path requires configuration if it's not already configured by your framework. For example, in a simple Rack app, add the following to your config.ru:

environment.context_class.class_eval do
  def asset_path(path, options = {})
    "/assets/#{path}"
  end
end

Rakefile

@@ -9,7 +9,7 @@ desc 'Dumps output to a CSS file for testing'
 task :debug do
   require 'sass'
   require './lib/bootstrap-sass/compass_functions'
-  require './lib/bootstrap-sass/rails_functions'
+  require './lib/bootstrap-sass/sass_functions'
   path = './vendor/assets/stylesheets'
   %w(bootstrap bootstrap-responsive).each do |file|
     engine = Sass::Engine.for_file("#{path}/#{file}.scss", syntax: :scss, load_paths: [path])

lib/bootstrap-sass/compass_functions.rb

@@ -1,14 +1,19 @@
 # This contains functions for use with a project *only* using Compass.
-
 module Sass::Script::Functions
-  # Define image_path for Compass to allow use of sprites without url() wrapper.
-  def image_path(asset)
-    if defined?(::Compass)
-      image_url(asset, Sass::Script::Bool.new(true))
+  def image_path(source, options = {})
+    if defined?(::Sprockets)
+      ::Sass::Script::String.new sprockets_context.image_path(source.value).to_s, :string
+    elsif defined?(::Compass)
+      image_url(source, Sass::Script::Bool.new(true))
     else
       # Revert to the old compass-agnostic path determination
-      asset_sans_quotes = asset.value.gsub('"', '')
+      asset_sans_quotes = source.value.gsub('"', '')
       Sass::Script::String.new("/images/#{asset_sans_quotes}", :string)
     end
   end
+
+  protected
+  def sprockets_context # :nodoc:
+    options[:custom][:sprockets_context]
+  end
 end

test/test_helper.rb

@@ -4,4 +4,4 @@ require 'test/unit'
 
 require 'sass'
 require 'lib/bootstrap-sass/compass_functions'
-require 'lib/bootstrap-sass/rails_functions'
+require 'lib/bootstrap-sass/sass_functions'