Cleanup URL handling in ScriptTask::load a bit.

This should make it clearer that we always pass a URL to Document::new, and
avoids an unnecessary unwrap() call.
auto merge of #3571 : Ms2ger/servo/rewrite-js-url, r=jdm

This should make it clearer that we always pass a URL to Document::new, and
avoids an unnecessary unwrap() call.