Http::Headers respects headers that are not prefixed with HTTP_
refactor, `Http::Headers` stores headers in env notation

Also:
cleanup, use consistent syntax for `Http::Header` and test.
allow headers and env to be passed in `IntegrationTest`.

Closes #6513.
`Http::Headers` respects dotted env vars, symbols, headers with numbers.
Merge pull request #9700 from senny/6513_integration_test_headers

rework passing headers and env variables in integration tests
Hidden field tags from nested hash

Add helper methods to convert flat or nested hashes into a sequence of hidden input fields to be submitted with the form.
Add CHANGELOG entry
Use Ruby 1.9 Hash syntax