Fix String#succ!, String#next!
Fix String#upcase!
Fix String#downcase!
Fix String#swapcase!
Fix String#capitalize!
Fix String#chop!
Fix String#chomp!
Fix String#reverse!
Fix String#clear
Fix String#<<
Fix String#tr!
Fix String#tr_s!
Merge pull request #595 from kachick/fix-string-check_frozen

Fix side effect methods in String when self is a frozen object