Make map_reduce @spec stricter
Make map_reduce @spec stricter

Signed-off-by: Mikhail S. Pobolovets <4a7321ca49ee84ca20631d03da5e994635b62455@gmail.com>

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