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

Author: Overbryd

lib/poison/encoder.ex

@@ -75,8 +75,11 @@ defimpl Poison.Encoder, for: Atom do
   def encode(false, _), do: "false"
 
   def encode(atom, options) do
-    Poison.Encoder.BitString.encode(Atom.to_string(atom), options)
+    mapping = Keyword.get(options, :force, %{})
+    value = Map.get(mapping, atom, Atom.to_string(atom))
+    Poison.Encoder.encode(value, Keyword.delete(options, :force))
   end
+
 end
 
 defimpl Poison.Encoder, for: BitString do

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