* Introduce `ActionDispatch::Http::Headers#add` to add a value to
a multivalued header.
* Move `Response#add_header` upstream: https://github.com/rack/rack/pull/957
* Match upstream `Response#have_header?` -> `#has_header?` name change.
Previously if you were looking for a given key, the header may incorrectly tell you that it did not exist even though it would return a valid value:
```ruby
env = { "CONTENT_TYPE" => "text/plain" }
headers = ActionDispatch::Http::Headers.new(env)
headers["Content-Type"]
# => "text/plain"
headers.key?("Content-Type")
# => false
```
This PR fixes that behavior by converting the key before checking for presence
The env hash passed to `Http::Headers#new` must be in env format.
Also be aware that the passed hash is modified directly.
docs and test-cases for setting headers/env in functional tests.
Follow up to #9700.
