PR#32381 added Rubocop's comments to some tests files in order to
exclude `Performance/RedundantMerge`.
Turn off `Performance` cops for tests files via `Exclude`
in `.rubocop.yml`.
Context https://github.com/rails/rails/pull/32381#discussion_r205212331
Use attr_reader/attr_writer instead of methods
method is 12% slower
Use flat_map over map.flatten(1)
flatten is 66% slower
Use hash[]= instead of hash.merge! with single arguments
merge! is 166% slower
See https://github.com/rails/rails/pull/32337 for more conversation
assert [1, 3].includes?(2) fails with unhelpful "Asserting failed" message
assert_includes [1, 3], 2 fails with "Expected [1, 3] to include 2" which makes it easier to debug and more obvious what went wrong
* 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.
