Add a note about the for_domain flag in the rdoc for CookieJar#add().
Add CookieJar#delete().
Simply use cookie.domain instead of cookie.domain_name.hostname.
Add a test for Cookie#accessed_at.
Omit expiration check in cookies().  Remove an unused variable.
CookieJar#cookie(): Allow call without an argument.
Update rdoc and comments.
Allow assigning nil to the cookie value to make an expiration cookie.

Cookie.new(): Make the value parameter can be omittable.
Add Cookie.cookie_value and Cookie.cookie_value_to_hash.