fix warning in Elixir 1.8

[KazuCocoa]

warning: the Collectable protocol is deprecated for non-empty lists. The behaviour of things like Enum.into/2 or "for" comprehensions with an :into option is incorrect when collecting into non-empty lists. If you're collecting into a non-empty keyword list, consider using Keyword.merge/2 instead. If you're collecting into a non-empty list, consider concatenating the two lists with the ++ operator.
  (elixir) lib/collectable.ex:83: Collectable.List.into/1
  (elixir) lib/enum.ex:1227: Enum.into_protocol/2
  (elixir) lib/enum.ex:1940: Enum."-reduce/3-lists^foldl/2-0-"/3
  (http_proxy) lib/http_proxy/play/paths.ex:49: HttpProxy.Play.Paths.path_patterns/1
  (http_proxy) lib/http_proxy/play/paths.ex:71: HttpProxy.Play.Paths.path_pattern?/1
  (http_proxy) lib/http_proxy/handle.ex:189: HttpProxy.Handle.play_conn/1
  test/http_proxy/http_test.exs:61: anonymous fn/3 in HttpProxy.HttpTest."test 'play responses agains particular request': number of 2"/1
  test/http_proxy/http_test.exs:59: HttpProxy.HttpTest."test 'play responses agains particular request': number of 2"/1
  (ex_unit) lib/ex_unit/runner.ex:355: ExUnit.Runner.exec_test/1
  (stdlib) timer.erl:166: :timer.tc/1
  (ex_unit) lib/ex_unit/runner.ex:306: anonymous fn/4 in ExUnit.Runner.spawn_test_monitor/4