Make sure optional callbacks can be specified multiple times
Make sure optional callbacks can be specified multiple times

ggcampinho/elixir,joshprice/elixir,kelvinst/elixir,pedrosnk/elixir,kelvinst/elixir,antipax/elixir,pedrosnk/elixir,ggcampinho/elixir,lexmag/elixir,kimshrier/elixir,kimshrier/elixir,beedub/elixir,michalmuskala/elixir,gfvcastro/elixir,lexmag/elixir,beedub/elixir,gfvcastro/elixir,elixir-lang/elixir,antipax/elixir