Previously memoization in `localtime` wasn't taking the `utc_offset`
parameter into account when returning a cached value. It now caches the
computed value depending on the `utc_offset` parameter, e.g:

    Time.zone = "US/Eastern"

    t = Time.zone.local(2016,5,2,11)
    # => Mon, 02 May 2016 11:00:00 EDT -04:00

    t.localtime(-7200)
    # => 2016-05-02 13:00:00 -0200

    t.localtime(-3600)
    # => 2016-05-02 14:00:00 -0100