Improve test coverage of Time#utc in 1.8 mode.
Add spec on Time#utc with years from 0 in >1.8 mode.
Fix Time.utc with years from 0 in >1.8 mode.
Merge pull request #2542 from razielgn/time-utc-year-zero

Fix Time.utc with years from 0 in >1.8 mode.