asgi spec violation: headers in scope can be an iterator

[devkral]

When I changed in an asgi framework the type of the headers in a scope to a special iterator which adheres the ASGI spec, uvicorn started to crash.

Here I will explain in detail what is wrong:

Let's assume headers in scope are a list: then everything works. This is the default. But the scope is mutable by the asgi app.
This means we cannot assume that that headers stay a list. E.g. middleware can replace it with an iterator

When headers aren't a list we have multiple problems:

1. consumed headers iterator by the asgi app. Uvicorn sees only an empty iterator.
2. uvicorn consumes the headers before the asgi app can receive them.
3. asgi app defines an incompatible __contains__ for an iterator producing the headers like stated in specs (lilya). This is what the PR fix asgi spec violation causing crash in lilya #2571 tries to fix.

My proposal is to save away the initial headers in scope and check if any of them is the close header. A middleware should not tamper here. I will make a new PR if ok.

[devkral]

ah yes before somebody says, hey, the code works. Here an counter-example

headers = iter([(1, 1), (2, 2), (3, 3)])
if (1, 1) in headers:
  print("works 1")
if (1, 1) in headers:
  # oops, this isn't executed anymore because of an exhausted iterator
  print("works 2")

The code seems to work properly but because of the exhaustion problem it doesn't work reliable. I will introduce an emergency fix for lilya soon.