Add url encoding option to URL.resolve

Given:

a.href = "over 9000!"

Then:

a.href === BASE_URL + '/over%209000!';

This commits adds an escape: bool option to URL.resolve which will escape the
path, query and fragment when true.

Also changes the Anchor, Image, Link and IFrame getSrc to escape. Escaping is
also used when navigating a frame.

src/browser/tests/element/html/anchor.html

@@ -245,3 +245,11 @@
   testing.expectEqual('', b.toString());
 }
 </script>
+
+<script id=url_encode>
+  {
+    let a = document.createElement('a');
+    a.href = 'over 9000!';
+    testing.expectEqual(testing.BASE_URL + 'element/html/over%209000!', a.href);
+  }
+</script>