Refactor how to use RSTR_NOFREE_P macro
str_replace: self should not be shared and nofree
String#clear: use String#replace to simple
Merge pull request #2387 from suzukaze/refactor-nofree-p

Refactor how to use RSTR_NOFREE_P macro
Merge pull request #2389 from ksss/str-mem-leaks

Fix memory leaks in String