Copy receiver's `MRB_STR_ASCII` flag in some methods of `String`

src/string.c

@@ -487,6 +487,7 @@ mrb_str_byte_subseq(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len)
     s->as.heap.ptr += beg;
     s->as.heap.len = len;
   }
+  RSTR_COPY_ASCII_FLAG(s, orig);
   return mrb_obj_value(s);
 }
 
@@ -902,6 +903,7 @@ mrb_str_times(mrb_state *mrb, mrb_value self)
     memcpy(p + n, p, len-n);
   }
   p[RSTR_LEN(str2)] = '\0';
+  RSTR_COPY_ASCII_FLAG(str2, mrb_str_ptr(self));
 
   return mrb_obj_value(str2);
 }
@@ -2837,6 +2839,7 @@ mrb_str_inspect(mrb_state *mrb, mrb_value str)
     }
   }
   mrb_str_cat_lit(mrb, result, "\"");
+  RSTR_COPY_ASCII_FLAG(mrb_str_ptr(result), mrb_str_ptr(str));
 
   return result;
 }