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
Move some railties changelog entries to the top [ci skip]