String#split more compatibility with CRuby

[ksss]

String#split with Regexp:

• splits between characters when regexp matches a zero-length string
• includes all captures in the result array
• does not include non-matching captures in the result array

[take-cheeze]

@mattn LGTM

[ksss]

Thank you for reviewing. I've resolved conflicts and added some test cases.

[mattn]

Perfect! Thank you.