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

lib/poison/encoder.ex

@@ -83,13 +83,15 @@ defprotocol Poison.Encoder do
   @typep indent :: non_neg_integer
   @typep offset :: non_neg_integer
   @typep strict_keys :: boolean
+  @typep force :: map
 
   @type options :: %{
     optional(:escape) => escape,
     optional(:pretty) => pretty,
     optional(:indent) => indent,
     optional(:offset) => offset,
     optional(:strict_keys) => strict_keys,
+    optional(:force) => force,
   }
 
   @spec encode(t, options) :: iodata
@@ -103,9 +105,20 @@ defimpl Poison.Encoder, for: Atom do
   def encode(true, _),  do: "true"
   def encode(false, _), do: "false"
 
+  def encode(atom, %{force: mapping} = options) when is_map(mapping) do
+    options = Map.delete(mapping, :force)
+    if Map.has_key?(mapping, atom) do
+      Map.get(mapping, atom)
+      |> Poison.Encoder.encode(options)
+    else
+      Encoder.BitString.encode(Atom.to_string(atom), options)
+    end
+  end
+
   def encode(atom, options) do
     Encoder.BitString.encode(Atom.to_string(atom), options)
   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