string.{c,rb}: fix type of return values from some methods as Ruby3.0

When the receiver is the instance of subclass of `String`.

- `String#each_char`
- `String#each_line`
- `String#partition`

1 files changed, 1 insertions, 1 deletions

diff --git a/mrbgems/mruby-string-ext/mrblib/string.rb b/mrbgems/mruby-string-ext/mrblib/string.rb
index 7544ac8f4..c61f427dc 100644
--- a/mrbgems/mruby-string-ext/mrblib/string.rb
+++ b/mrbgems/mruby-string-ext/mrblib/string.rb
@@ -148,7 +148,7 @@ class String
       m = n + sep.size
       [ slice(0, n), sep, slice(m, size - m) ]
     else
-      [ self, "", "" ]
+      [ self[0..-1], "", "" ]
     end
   end