Hash#add_to_key: configure behavior via keywords
Hash#add_to_key: configure behavior via keywords

Warning: one annoying thing with the way the ruby parser works is that
in `foo.add_to_key(:foo, :bar, {plom: :plim})`, plom: is interpreted as
a keyword. So when passing a hash of symbols, one should specify a dummy
keyword.

DamienRobert/drain