Fix incorrect `MRB_STR_ASCII` flag update in `mrb_str_dump`

### Example (with `MRB_UTF8_STRING`)

  ```ruby
  s = "\u3042"
  p s.size
  s.dump
  p s.size
  ```

#### Before this patch:

  ```
  1
  3
  ```

#### After this patch:

  ```
  1
  1
  ```

src/string.c

@@ -1388,8 +1388,13 @@ str_escape(mrb_state *mrb, mrb_value str, mrb_bool inspect)
   }
   mrb_str_cat_lit(mrb, result, "\"");
 #ifdef MRB_UTF8_STRING
-  mrb_str_ptr(str)->flags |= ascii_flag;
-  mrb_str_ptr(result)->flags |= ascii_flag;
+  if (inspect) {
+    mrb_str_ptr(str)->flags |= ascii_flag;
+    mrb_str_ptr(result)->flags |= ascii_flag;
+  }
+  else {
+    RSTR_SET_ASCII_FLAG(mrb_str_ptr(result));
+  }
 #endif
 
   return result;