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
```
MODIFY
src/string.c
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
+ 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;

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
  ```