Increment the env.version with asset_host and relative_url_root

This way such configurations will affect the asset fingerprints and
the environment cache, since they might affect the assets contents.

Author: lucasmazza

lib/sprockets/railtie.rb

@@ -66,8 +66,16 @@ def configure(&block)
 
       manifest_path = File.join(app.root, 'public', config.assets.prefix)
 
-      unless config.assets.version.blank?
-        app.assets.version += "-#{config.assets.version}"
+      # Configuration options that should invalidate
+      # the Sprockets cache when changed.
+      version_fragments = [
+        config.assets.version,
+        config.action_controller.relative_url_root,
+        config.action_controller.asset_host
+      ].compact.join('-')
+
+      if version_fragments.present?
+        app.assets.version += "-#{version_fragments}"
       end
 
       # Copy config.assets.paths to Sprockets

test/test_railtie.rb

@@ -109,6 +109,18 @@ def test_version
     assert_equal "test-v2", env.version
   end
 
+  def test_version_fragments
+    app.configure do
+      config.assets.version = 'v2'
+      config.action_controller.asset_host = 'http://some-cdn.com'
+      config.action_controller.relative_url_root = 'some-path'
+    end
+    app.initialize!
+
+    assert env = app.assets
+    assert_equal "test-v2-some-path-http://some-cdn.com", env.version
+  end
+
   def test_configure
     app.configure do
       config.assets.configure do |env|