Note about read timeout errors and max_retries
Add DeprecationWarnings to inform users of plans

After a long discussion in IRC and on several issues, the developers of
requests have decided to remove specific functions from requests.utils
in version 3.0.0. To give users ample time to prepare for this, we've
added DeprecationWarnings long in advance. See also the planning of this
in issue #2266.
Update documentation about max_retries to reflect code
Close sessions created in the functional API

This is related to #1882 and #1685. By calling close on the session, we
clear the PoolManager operated by the Session and close all sockets.

Fixes #1882
Partially-fixes #1685
Added test for overriding Content-Length.
Fixed.
Fix HTTPDigestAuth not to treat non-file as a file

Ensure pos is set to None when the body is not a file
so that HTTPDigestAuth detects the type of the body correctly.
Merge pull request #2333 from akitada/reinit-pos

Fix HTTPDigestAuth not to treat non-file as a file
Merge pull request #2326 from sigmavirus24/close-functional-sessions

Close sessions created in the functional API
Merge pull request #2332 from asnelzin/fix-2329

Add overriding Content-Length
Merge pull request #2308 from kevinburke/read-timeout-retry

Note about read timeout errors and max_retries
Merge pull request #2309 from sigmavirus24/add-deprecation-warnings

Add DeprecationWarnings to inform users of plans