fix asgi spec violation causing crash in lilya

[devkral]

Summary

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

Detail:

The asgi spec only specifies that the headers are an iterable. Uvicorn assumes that the headers in scope are a list. This causes a crash at least in lilya and can cause hard to debug bugs because of exhausted iterators

[Kludex]

I don't think it assumes that.

if (1, 1) in iter([(1, 1), (2, 2), (3, 3)]):
    print('It works with iterators...')

Please post some reference to the issue.

[devkral]

it works once:

foo = iter([(1, 1), (2, 2), (3, 3)])
if (1, 1) in foo:
  print("works 1")
if (1, 1) in foo:
  print("works 2")

The problem are the iterator is exhausted. I referenced already a framework which has bugs because of this behavior.

Note: I introduced the change which caused the bug.