Fix precompile asset_host test

Fix for relative url root in assets

Move asset tests to assets_test file

fixed the failing generators_test in railties.

Set relative url root in assets when controller isn't available for Sprockets

fix https://github.com/rails/rails/issues/2947 (with testcase)

See https://github.com/rails/sass-rails/issues/42 for details

  * A collection of Routes
  * The Router, which consults the collection of routes
  * A formatter that consults the collection of routes

This shouldn't ever happen unless people are doing something
particularly weird, but adding a deprecation in case there are bugs not
caught by our tests.