requests: incorrectly extends callers headers dict

[agadsby]

Using MicroPython v1.24.1 on 2024-11-29; Raspberry Pi Pico W with RP2040

The trivial test program below works correctly on Python but fails on Micropython :

import requests
# test to demonstrate that micropython request incorrectly modifies caller's headers dict
headers = {}

response = requests.get(
            url="http://www.google.com",
            headers=headers,
            data="string",
        )
print(headers)
assert headers == {}, "requests returned changed headers to parent"

Fix is equally trivial but my github skills are limited.

After these lines:

micropython-lib/python-ecosys/requests/requests/__init__.py

Lines 47 to 48 in e4cf095

	if headers is None:
	headers = {}

Add

    else:
        headers = headers.copy()

to take a local copy for subsequent use in requests().