Ensure we properly underscore acronyms followed by paths

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/mix/lib/mix/utils.ex

@@ -171,7 +171,7 @@ defmodule Mix.Utils do
     <<to_lower_char(h)>> <> do_underscore(t, h)
   end
 
-  defp do_underscore(<<h, t, rest :: binary>>, _) when h in ?A..?Z and not t in ?A..?Z do
+  defp do_underscore(<<h, t, rest :: binary>>, _) when h in ?A..?Z and not (t in ?A..?Z or t == ?.) do
     <<?_, to_lower_char(h), t>> <> do_underscore(rest, t)
   end
 

lib/mix/test/mix/utils_test.exs

@@ -37,6 +37,7 @@ defmodule Mix.UtilsTest do
     assert Mix.Utils.underscore("FoBaZa") == "fo_ba_za"
     assert Mix.Utils.underscore("Foo.Bar") == "foo/bar"
     assert Mix.Utils.underscore(Foo.Bar) == "foo/bar"
+    assert Mix.Utils.underscore("API.V1.User") == "api/v1/user"
     assert Mix.Utils.underscore("") == ""
     assert Mix.Utils.underscore("..") == ".."
     assert Mix.Utils.underscore("/..") == "/.."