Added missing deleted? method to cookie jar.

[jwoertink]

Purpose

Fixes #963

Description

In the guides we make mention of a cookies.deleted?() method that never actually existed. I'm not sure where it came from (probably me), but this adds that method in.

Deleting cookies is weird because to delete a cookie, you set the value to an empty string, and the expire time to something in the past. It's actually up to the individual browsers to remove that cookie. This deleted? method will return true if the cookie is both expired? && value.blank? otherwise it's just false.

Side note: in the guides, we also make mention of session.deleted?(:name). I decided to not implement this because if a session key is deleted, then it's just nil. I'm ok with adding it for consistency in the API, but initially I didn't think it's necessary.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[paulcsmith]

Looks good! And I agree on the session.deleted?. Since it is stored as a JSON object it is basically the same as checking if it is nil so we can remove that from the guides