• Add t/20_unknown.t tests from JSON::PP, extended.
  
• Fix convert_blessed, disallow invalid JSON. #46 (reported by Dan Book)
  convert_blessed returns now always a string, even for numbers.
  
• Fix encountered GLOB error message (still in JSON::XS, and JSON::PP took
  over the wrong message also).
  
• Fixed regression of immediate raw values for null/true/false to be
  modifiable again. #45. Broken with 3.0201 - 3.0204.
  (reported by Thomas Sibley and Karen Etheridge)
  This caused failues in Test::JSON and App::RecordStream.