Fix encode_to_iodata/1 not matching encode/1 behaviour (#397)

Only encode/1 was applying transform in the outer struct

lib/protobuf/encoder.ex

@@ -8,7 +8,9 @@ defmodule Protobuf.Encoder do
 
   @spec encode_to_iodata(struct()) :: iodata()
   def encode_to_iodata(%mod{} = struct) do
-    encode_with_message_props(struct, mod.__message_props__())
+    struct
+    |> transform_module(mod)
+    |> do_encode_to_iodata()
   end
 
   @spec encode(struct()) :: binary()
@@ -19,6 +21,10 @@ defmodule Protobuf.Encoder do
     |> IO.iodata_to_binary()
   end
 
+  defp do_encode_to_iodata(%mod{} = struct) do
+    encode_with_message_props(struct, mod.__message_props__())
+  end
+
   defp encode_with_message_props(
          struct,
          %MessageProps{syntax: syntax, field_props: field_props, ordered_tags: ordered_tags} =
@@ -148,15 +154,15 @@ defmodule Protobuf.Encoder do
   defp encode_from_type(mod, msg) do
     case msg do
       %{__struct__: ^mod} ->
-        encode_to_iodata(msg)
+        do_encode_to_iodata(msg)
 
       %other_mod{} = struct ->
         raise Protobuf.EncodeError,
           message:
             "struct #{inspect(other_mod)} can't be encoded as #{inspect(mod)}: #{inspect(struct)}"
 
       _ ->
-        encode_to_iodata(struct(mod, msg))
+        do_encode_to_iodata(struct(mod, msg))
     end
   end