Implement middleware to handle multi set cookie

[FabienArcellier]

Related to #44
Handles the transmission of multiple cookies returned by a server as multiple set-cookie headers.

Request merges the set-cookie headers into one. The default behavior is ok
if the server returns only one cookie per http response.

The browser will receive only one cookie.

{
    Set-Cookie: hello=world; Expires=Wed, 21 Oct 2015 07:28:00 GMT, world=hello
}

instead

{
    Set-Cookie: hello=world; Expires=Wed, 21 Oct 2015 07:28:00 GMT
    Set-Cookie: world=hello
}

more about this behavior

• urllib3/urllib3@d8013cb
• https://stackoverflow.com/a/57131320
• comma in case: join multiple Set-Cookie Headers psf/requests#3957 (comment)