Fix String#chars for 2.0

Avoid &block from following commits.

* ad47350b11776bf0a156554c3118d6f3c643c400
* 1211ffb10c2fda7d5f5bd01a09c77a69a464b288
Fix String#bytes for 2.0
Fix String#lines for 2.0
Merge pull request #2456 from kachick/2.0-string-chars,bytes,lines

Fix String#{chars,bytes,lines} for 2.0