Update __init__.py to prevent changes to caller's headers dict

[agadsby]

The Micropython version of urequests behaves differently to Python's in that if a headers dict is passed in it gets extended by entries such as Content-Length that are added to the caller's dict.

Change is to take a local copy of any headers passed in to urequests() otherwise the caller's headers gets incorrectly extended.

The snip below illustrates the problem.

import urequests as requests
# test to demonstrate that micropython urequest incorrectly modifies passed headers dict
headers = {}

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