Fix `String.slice error`
Fix `String.slice error`

    iex(76)> String.slice("string", 0..-2)
    nil # you see here is the issue, it shouldn't return `nil`
    iex(77)> String.slice("string", 1..-2)
    "trin"
    iex(78)> String.slice("string", -5..-2)
    "trin"

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