Fix: Handle both uppercase and lowercase Content-Type header keys

[yyyhakuna]

Summary

Fix for issue #7234: Quoted charset values are not detected by get_encoding_from_headers

Problem

The get_encoding_from_headers function was only checking for lowercase content-type key, but HTTP headers are often passed with uppercase Content-Type. This caused the function to return None when headers had uppercase keys, even when a valid charset was present.

Solution

Check for both content-type (lowercase) and Content-Type (uppercase) header keys.

Test

from requests.utils import get_encoding_from_headers

headers = {"Content-Type": 'text/html; charset="utf-8"'}
print(get_encoding_from_headers(headers))  # Now returns 'utf-8' instead of None

Fixes

#7234