Add remote url test case

Author: yuezk

test/fixtures/expected/test-remote.css

@@ -1,3 +1,7 @@
 body {
     background: url(http://127.0.0.1:10010/images/test.png?v=e19ac7dee6);
-}
+}
+
+body {
+    background: url(//127.0.0.1:10010/images/test.png?v=e19ac7dee6);
+}

test/fixtures/test-remote.css

@@ -1,3 +1,7 @@
 body {
     background: url(http://127.0.0.1:10010/images/test.png);
-}
+}
+
+body {
+    background: url(//127.0.0.1:10010/images/test.png);
+}