Fix to_string for Version with numeric pre

Signed-off-by: Eric Meadows-Jönsson <eric.meadows.jonsson@gmail.com>

lib/elixir/lib/version.ex

@@ -519,10 +519,21 @@ end
 
 defimpl String.Chars, for: Version do
   def to_string(version) do
-    pre = unless Enum.empty?(pre = version.pre), do: "-#{pre}"
+    pre = pre(version.pre)
     build = if build = version.build, do: "+#{build}"
     "#{version.major}.#{version.minor}.#{version.patch}#{pre}#{build}"
   end
+
+  defp pre([]) do
+    ""
+  end
+
+  defp pre(pre) do
+    Enum.map(["-"] ++ pre, fn
+      int when is_integer(int) -> Integer.to_string(int)
+      string when is_binary(string) -> string
+    end)
+  end
 end
 
 defimpl Inspect, for: Version do

lib/elixir/test/elixir/version_test.exs

@@ -72,6 +72,7 @@ defmodule VersionTest do
     assert V.parse("1.0.0-dev") |> elem(1) |> to_string == "1.0.0-dev"
     assert V.parse("1.0.0+lol") |> elem(1) |> to_string == "1.0.0+lol"
     assert V.parse("1.0.0-dev+lol") |> elem(1) |> to_string == "1.0.0-dev+lol"
+    assert V.parse("1.0.0-0") |> elem(1) |> to_string == "1.0.0-0"
   end
 
   test "invalid match" do