MRI: changes to mkxp's Marshal::load monkey patc

- force string encoding to UTF-8 only when it's ASCII-8BIT (force only valid UTF-8 strings in future?)

- support original Marshal::load proc (2nd arg)
MRI: changes to mkxp's Marshal::load monkey patch

- force string encoding to UTF-8 only when it's ASCII-8BIT (force only valid UTF-8 strings in future?)

- support original Marshal::load proc (2nd arg)