Add Rack::Lint to ActionDispatch::Callbacks tests

[nunosilva800]

Motivation / Background

To ensure Rails is and remains compliant with the Rack 3 spec we can add Rack::Lint to the Rails middleware tests.

This adds additional test coverage for ActionDispatch::Callbacks by validating that its input and output follow the Rack SPEC.

• https://github.com/skipkayhil/tmp-rails/issues/1

Detail

ActionDispatch::Callbacks makes no changes to the input or output of the Rack app itself, so these tests are trivial.

Additional information

The "rack.input" => StringIO.new("") header value raised the following error:

Rack::Lint::LintError: rack.input #<StringIO:0x00007fd7513fe550> does not have ASCII-8BIT as its external encoding

Since this header is not required for the test, it is now removed.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.