add BSON.Binary{subtype: :binary_old} encoder test

Author: skinandbones

lib/bson/encoder.ex

@@ -17,6 +17,12 @@ defmodule BSON.Encoder do
   def encode(:BSON_max),
     do: ""
 
+  def encode(%BSON.Binary{binary: binary, subtype: :binary_old}) do
+    subtype = subtype(:binary_old)
+    size = IO.iodata_length(binary)
+    [<<size + 4::int32>>, subtype, <<size::int32>>, binary]
+  end
+
   def encode(%BSON.Binary{binary: binary, subtype: subtype}) do
     subtype = subtype(subtype)
     [<<IO.iodata_length(binary)::int32>>, subtype | binary]