Fixed some minor code "derps"

I can't believe I never noticed the missing `break` statements in
`_getExpiresDate` before now. Good thing passing a Date instance into
the constructor for a new Date instance just ends up creating a copy of
the original... Though, unit tests would have caught it if falling
through caused a bug, so that's reassuring (hooray TDD!). I should
probably start linting my JS.
Made a minor update suggested by JSHint