check a̲l̲l̲ control chars from RFC5545 §3.3.11. (below \x80, anyway)

fixes https://github.com/fruux/sabre-vobject/issues/134
Merge pull request #136 from mirabilos/fix-repairing-control-chars

check a̲l̲l̲ control chars from RFC5545 §3.3.11. (below \x80, anyway)