Quote 'y'/'Y'/'n'/'N' as names too (to avoid problems with Boolean keys)

[cowtowncoder]

(note: replacement of #225 targeting 2.12 branch)

Looks like #68 left out single-letter boolean values ("Y"/"y"/"N"/"n") wrt quoting of names, presumable for aesthetic reasons.
It seems better to quote these, however, since they are already quoted when output as values (see MUST_QUOTE_VALUES in YAMLGenerator).
Change is planned for 2.12.0; we can still revisit this after 2.12.0-rc1 if there are actual problems introduced (and if so possibly add Yet Another YAMLGenerator.Feature if need be).