Remove some 'implementation detail' headers from the public API.

Application code doesn't need to worry about headers like
'Connection' or 'Transfer-Encoding', so don't include them
in the API.
Merge pull request #391 from square/jwilson_0101_headers

Remove some 'implementation detail' headers from the public API.
Fix a bug where HTTPS POSTS were always buffered.

https://code.google.com/p/android/issues/detail?id=64348
Merge pull request #393 from square/jwilson_0102_delegate_everything

Fix a bug where HTTPS POSTS were always buffered.
Make redirects work with the async API.
Merge pull request #395 from square/jwilson_0102_async_redirects

Make redirects work with the async API.
Another round of header APIs cleanup.

This promotes Headers to a public API from the internal package.
It moves some of its methods to OkHeaders, which has been renamed
from SyntheticHeaders.

Making the Headers class public API makes it possible to remove
more exotic APIs from Request and Response like the APIs to track
headers by index rather than by name.
Merge pull request #394 from square/jwilson_0102_headers_apis

Another round of header APIs cleanup.