The field reader dropped the value "Off" for every field kind, so a
text field, combo box or single-select list set to "Off" read as empty
and exported an empty value to CSV, text, XML, XFDF and FDF, although
the document stored it. "Off" is now treated as no value only for
check boxes and radio buttons, where it is the off state.
Adds tests::a_literal_off_value_reads_as_off_outside_buttons, which
fails before this change.