Change from each to each_value in http/parameters since we don't use key
Http::Headers respects headers that are not prefixed with HTTP_
refactor, `Http::Headers` stores headers in env notation
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.