allow to map atoms to other values to handle null -> nil

Author: Overbryd

lib/poison/encoder.ex

@@ -74,6 +74,11 @@ defimpl Poison.Encoder, for: Atom do
   def encode(true, _),  do: "true"
   def encode(false, _), do: "false"
 
+  def encode(atom, %{force: mapping} = options) do
+    value = Map.get(mapping, atom, Atom.to_string(atom))
+    Poison.Encoder.encode(value, Map.delete(options, :force))
+  end
+
   def encode(atom, options) do
     Poison.Encoder.BitString.encode(Atom.to_string(atom), options)
   end

test/poison/encoder_test.exs

@@ -8,6 +8,8 @@ defmodule Poison.EncoderTest do
     assert to_json(true) == "true"
     assert to_json(false) == "false"
     assert to_json(:poison) == ~s("poison")
+    assert to_json(:null) == ~s("null")
+    assert to_json(:null, force: %{null: nil}) == "null"
   end
 
   test "Integer" do