Fix some warnings from mix

lib/geef/object.ex

@@ -44,10 +44,10 @@ defmodule Geef.Object do
     Tree.get(tree, key)
   end
 
-  def get(tree = %Object{type: :tree}, key) do
+  def get(tree = %Object{type: :tree}, key, default \\ nil) do
     case fetch(tree, key) do
       {:ok, entry} -> entry
-      {:error, _} -> nil
+      {:error, _} -> default
     end
   end
 
@@ -56,4 +56,8 @@ defmodule Geef.Object do
     raise ArgumentError
   end
 
+  def pop(_tree, _key) do
+    raise ArgumentError
+  end
+
 end