xml() method is invented

[developit]

Looks like I picked up baggage in my understanding of Fetch and invented an .xml() method on Response that doesn't actually exist. We'll need to remove that since it's not spec compliant.

Originally noted here: tkh44/holen#3

[vesparny]

Here the specs

https://developer.mozilla.org/en-US/docs/Web/API/Response

[developit]

lovely! no mention of xml on that page, time to remove it :)