gh-158225: Fix wsgiref Headers __setitem__ validation order

[Tony123-tech]

• Issue: wsgiref.headers.Headers.__setitem__ deletes the old header before validating the new value #158225

[sergiolpzgmz]

Hi!

I reviewed and tested this change locally.

Specifically, I checked that when validation of the new header name or value fails, the existing header value is preserved:

from wsgiref.headers import Headers

h = Headers([("Content-Type", "text/html")])

try:
    h["Content-Type"] = True
except AssertionError:
    pass

assert h.get("Content-Type") == "text/html"

I also checked the case where validation of the header name fails because it contains a control character:

from wsgiref.headers import Headers

h = Headers([("Content-Type", "text/html")])

try:
    h["Content-\x00Type"] = "application/json"
except ValueError:
    pass

assert h.get("Content-Type") == "text/html"

In both cases, the existing header remains unchanged.

I also ran the relevant test suite:

$ ./python -m test test_wsgiref

== Tests result: SUCCESS ==

1 test OK.
Total tests: run=47
Result: SUCCESS

I didn't find any issues with the change in these checks.