[bug] Fixing buffered streams to proxy properly

Fixes #109, #125
[fix] Opt in only content-type and content-length headers
[minor] Adding a comment for request header fixup
Merge pull request #126 from rackspace/newBufferedStream

[Rackspace] Refactoring Inline Authentication for Streams (Upload/Download)
[version] pkgcloud version 0.7.2