Document the new behavior of Praxis where headers are backed by Hashes, and the fact that their keys are case-sensitive (i.e. need to use them in the same string-case that you defined them)

Signed-off-by: Josep M. Blanquer <blanquer@rightscale.com>
Merge pull request #53 from rightscale/docs_headers_as_hashes

Document the new behavior of Praxis where headers are backed by Hashes