Add Hash#invert
Refactor mrb_str_capitalize_bang()
Refactor mrb_str_downcase_bang()
Refactor mrb_str_split_m()
Refactor mrb_str_upcase_bang
Fix indents in mrb_str_inspect() of string.c
Merge pull request #2192 from suzukaze/add-hash.invert

Add Hash#invert
Merge pull request #2193 from suzukaze/refactor-string.c

Use mrb_bool instead of mrb_int  in string.c
Merge pull request #2195 from suzukaze/fix-indent-string.c

Fix indents in mrb_str_inspect() of string.c