Merge pull request #1107 from Lukasa/master

Correct broken cookies example
Remove support for certifi

As per #1105, certifi is being end-of-lifed. Requests will use either
its own vendored bundle, or possibly (when packaged with OS distributions)
an externally packaged bundle, which can be enabled by patching
requests.certs.where().
Merge pull request #1110 from slingamn/certifi_eol

Remove support for certifi
Fixed proxy requests to pool connections.
Merge pull request #1114 from vlaci/master

Keep-alive support for http proxies
Ensure Content-Length is a string.