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)
follow-up: style adjustments (see GH-2)
Merge pull request #2 from cremno/mri-marshal-utf8proc

MRI binding: changes to Marshal::load mkxp's custom proc