Pickling of Session and HTTPAdapter + a test

This is for issue #1088
Merge pull request #1243 from darjus-amzn/master

Pickling of Session and HTTPAdapter + a test
Fixed some pickeling issues with HTTPAdapter and Session

Added trust_env and stream to Session.__attrs__. Initialize
self._pool_connections and self._pool_maxsize in HTTPAdapter.
Merge pull request #1244 from jajadinimueter/master

Fixed further pickeling issues in Session and HTTPAdapter
Add failing test case for #1228
Use session cookies when following redirects

When a redirect was followed, only the cookies set by the initial
response were used in the follow-up request. Fixes #1228.
Do not pass cookies to resolve_redirects

SessionRedirectMixin is extending Session, so we can just use
self.cookies.