Headers iterator must allow modification during iteration

The WPT test `headers/headers-basic.any.js` requires us to implement the following capabilities:
- Iteration skips elements removed while iterating
- Removing elements already iterated over causes an element to be skipped during iteration
- Appending a value pair during iteration causes it to be reached during iteration
- Prepending a value pair before the current element position causes it to be skipped during iteration and adds the current element a second time

We can't currently do this because our iterator just works on a copy of the header list. We need a 'live' iterator with a cursor instead, in order to match expected behaviour.