Allow binary fields to be nil

Author: mweidner037

lib/mongo_ecto.ex

@@ -491,6 +491,9 @@ defmodule Mongo.Ecto do
   end
 
   defp load_binary(%BSON.Binary{binary: binary}), do: {:ok, binary}
+
+  defp load_binary(nil), do: {:ok, nil}
+
   defp load_binary(_), do: :error
 
   defp load_objectid(%BSON.ObjectId{} = objectid) do
@@ -587,6 +590,7 @@ defmodule Mongo.Ecto do
   defp dump_binary(binary, subtype) when is_binary(binary),
     do: {:ok, %BSON.Binary{binary: binary, subtype: subtype}}
 
+  defp dump_binary(nil, _), do: {:ok, nil}
   defp dump_binary(_, _), do: :error
 
   defp dump_objectid(<<objectid::binary-size(24)>>) do