Do not set headers with None value

- Regardless of whether they are on the session or not
- Fixes #1920
Charade -> Chardet and Add cacert.pem license

- Charade is gone, long live Chardet.
- cacert.pem is now taken wholesale from Mozilla so we need to display that
  itis licensed under the MPL2.0
Add timeout to stream with testing

Fixes Issue #1803
Fix styling issues with add timeout to stream with testing
Remove unnecessary bits from cacert notice section
Merge pull request #1934 from kennethreitz/update-notice

Charade -> Chardet and Add cacert.pem license
Merge pull request #1921 from sigmavirus24/fix-session-header-none

Do not set headers with None value
Merge pull request #1935 from cjstapleton/master

Add timeout to stream with testing
Timeout documentation changes.
Update urllib3 to e2353dc.
Read Accept-Encoding header from urllib3.