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"

Author: jsvisa

lib/elixir/lib/string.ex

@@ -913,7 +913,7 @@ defmodule String do
       last = total + last
     end
 
-    if first > 0 do
+    if first >= 0 do
       do_slice(next_grapheme(string), first, last, 0, "")
     end
   end