Since `requests` creates a comma-separated list for any duplicated headers, this causes a problem when `expires=...` is between two `Set-Cookie` header values. `BaseCookie.__parse_string()` in https://github.com/python/cpython/blob/master/Lib/http/cookies.py, in that case, will just give up, since it thinks it was given an invalid cookie. The fix is to replace the comma at the end of each trailing `expires=...` with a semicolon. Inside `BaseCookie.__parse_string()`, before the `while` loop, all that should be needed is to add this:
```
str = re.sub('(=\w{3},\s[\w\d\s-]{9,11}\s[\d:]{8}\sGMT),', r'\1;', str)
```

whoops, the first string should also be a raw string, i.e. `r'...'`

Here's a simple example. This is correctly parsed via a `SimpleCookie().load()` call:
'key1=val1, key2=val2'

but this fails to parse:
'key1=val1; expires=Wed, 21 Oct 2015 07:28:00 GMT, key2=val2'

My suggested fix:
str = re.sub(r'(=\w{3},\s[\w\d\s-]{9,11}\s[\d:]{8}\sGMT),', r'\1;', str)

will convert the comma separator to a semicolon, so this will correctly parse:
'key1=val1; expires=Wed, 21 Oct 2015 07:28:00 GMT; key2=val2'