Use TLS 1.2

[jobala]

Overview

Enforce Requests to use an arbitrary TLS version -- TLS 1.2

Notes

We can enforce Requests to use an arbitrary TLS version through a HTTPAdapter. However, we can only mount one HTTPAdapter at any time. We are already using a HTTPAdapter to implement the middleware pipeline. To support TLS 1.2 and middlewares at the same time, we need to write a HTTPAdapter that enforces use of TLS 1.2 and supports a middleware pipeline.