Do not set headers with None value

- Regardless of whether they are on the session or not
- Fixes #1920
Revert "Proxy urls should have explicit schemes."

This reverts commit 840540b6b1f07ef87faab73392c03fbef0dcc9fe.

Conflicts:
	requests/adapters.py
	requests/utils.py
Default proxy scheme to http.
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.
Add documentation about decode_unicode.
Always honor decode_unicode, even when _content is present.

--HG--
extra : amend_source : 25977a1227b163d49bf2e1aec6aa448e5cd3be8a
Add tests for decode_unicode
Removed compress from accepted encodings.
Ensure that .raw() is present after unpickling.

Some people will assume that .raw() is present, and they shouldn't get
AttributeErrors when they make that assumption on a pickled Response.
However, @kennethreitz has asked that we not be too dependent on
urllib3. For that reason, set to None.
Split get_environ_proxies into two methods.

This makes it possible to get at the no_proxy logic separately.
Move auth rebuild to its own method.
Add method for rebuilding proxy configuration.

This includes auth.
Merge pull request #1949 from Lukasa/pickle

Pickled Responses should include a None value for the raw attribute