#### Before this patch:

  ```
  $ echo 'p true' | bin/mrdb /dev/null
  (/dev/null:1) mruby application exited.
  FrozenError: can't modify frozen String
  (-:0)
  ```

#### After this patch:

  ```
  $ echo 'p true' | bin/mrdb /dev/null
  (/dev/null:1) $1 = true
  (/dev/null:1)
  ```