Improve ActionCable's TestCookieJar interface

[justinko]

Fixes #51914

Motivation / Background

This change brings ActionCable::Connection::TestCookieJar in alignment with ActionDispatch::Cookies::CookieJar in regards to setting the cookie value.

Before:

cookies[:foo] = { value: "bar" }
puts cookies[:foo] # => { value: "bar" }

After:

cookies[:foo] = { value: "bar" }
puts cookies[:foo] # => "bar"

Additional information

Technically this is a breaking change but I don't know why the user would depend on an incorrect interface.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes 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.

[lzell]

Thank you @justinko and @rafaelfranca for tackling that so quickly!

[zzak]

@justinko Would you mind adding a changelog? The Action Cable changelog is usually pretty short, so no need to be conservative IMO.

[justinko]

@justinko Would you mind adding a changelog? The Action Cable changelog is usually pretty short, so no need to be conservative IMO.

#51979