Fix testing with simplecov.
Use case-when.
Improve rdoc.
Perform acceptance check in CookieJar#add instead of origin=.

- Cookie#acceptable? is added, which is called by such methods as
  Cookie.parse and CookieJar#add.

- Cookie#origin= no longer raises ArgumentError just because it
  conflicts with the domain.

- Cookie#origin= raises ArgumentError if it is given an object that is
  not URI or string-like.
Cookie#domain=: Fix handling of an empty string and let it accept nil.
Bump VERSION to 1.0.0.pre9.
Emulate behavior of URI() of ruby >=1.9 where it raises ArgumentError.
Avoid a bug of ruby 1.8.