Optimize Date._iso8601

* Passes test__iso8601

* ruby 2.0.0p247
  Date._iso8601 9.046 µs/i ± 0.053 ( 0.6%) <=>    110548 ips
* before
  Date._iso8601 72.35 µs/i ± 24.68 (34.1%) <=>     13821 ips
* after
  Date._iso8601 5.185 µs/i ± 0.065 ( 1.3%) <=>    192847 ips

* Date.iso8601 is twice faster, bottleneck seems now #complete_frags