Combine two conditionals in ScriptTask::load.

Now that the code lives in the same function, I see no reason for them to
remain separate.
Move the load response into the HTMLInput enum.

It is None iff the parser input is an InputString variant, so it makes more
sense to pass it in the same enum.
Remove the url from the InputUrl variant.

This url is the pre-redirect url, which is not particularly meaningful, and it
is used only in a panic message.
auto merge of #4293 : Ms2ger/servo/script_task, r=Manishearth