Expect: 100-Continue support

[Nathaniel J. Smith]

I learned from here that botocore does some strange things to patch urllib3 to support Expect: 100-continue.

This is definitely not our first priority, but native support for Expect: 100-continue support should be fairly straightforward using h11 – probably easier than supporting botocore's monkeypatching! – so wanted to make a note here so I don't forget.