Revert "Stop using decode_pair/2 in tests"

We still need to test the old implementation.

test/plug/conn/query_test.exs

@@ -181,9 +181,7 @@ defmodule Plug.Conn.QueryTest do
     end
 
     defp decode_pair(pairs) do
-      pairs
-      |> Enum.reduce(Plug.Conn.Query.decode_init(), &Plug.Conn.Query.decode_each/2)
-      |> Plug.Conn.Query.decode_done()
+      Enum.reduce(Enum.reverse(pairs), %{}, &Plug.Conn.Query.decode_pair(&1, &2))
     end
   end
 end