RequestsCookieJar._find_on_duplicates wrongly fails on cookie with empty value

Getting the value of a cookie which is set without a value by a website raises a KeyError. I don't know if this is by design. Browsers seem to accept these kind of cookies, even though this behaviour seems like bad practice.

Expected Result

An empty string.

Actual Result

RequestsCookieJar._find_no_duplicates raises a KeyError

Reproduction Steps

In ipython:

import requests
resp = requests.get("https://www.pib-insurance.com/")
resp.cookies.get_dict() # pretty print cookies
resp.cookies["firstname"]

Fix

I think last if in _find_no_duplicates (line 397 of cookies.py) should be if toReturn is not None: to accommodate for cookies without values.

Thanks!

Start in cookies.py at RequestsCookieJar._find_no_duplicates and reproduce the failure with the IPython example from the issue. Verify the empty-value cookie lookup returns an empty string rather than raising KeyError, and check the existing cookie behavior around this method before considering the change complete.