Allow for pre-encoded JSON during encoding

This change allows users to insert pre-encoded JSON `iodata()` anywhere
there can be a valid `json_value()` (i.e., anywhere except object keys).
This approach occurred to me looking at PR davisp#139 from @dhull. The
technical difference being that this approach does not copy the given
`iodata()` into the output and instead re-uses the same input term as
part of the output `iodata()`.

c_src/encoder.c

@@ -859,8 +859,11 @@ encode_iter(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
             }
         } else if(enif_get_tuple(env, curr, &arity, &tuple)) {
             if(arity != 1) {
-                ret = enc_obj_error(e, "invalid_ejson", curr);
-                goto done;
+                if(!enc_unknown(e, curr)) {
+                    ret = enc_error(e, "internal_error");
+                    goto done;
+                }
+                continue;
             }
             if(!enif_is_list(env, tuple[0])) {
                 ret = enc_obj_error(e, "invalid_object", curr);

src/jiffy.erl

@@ -173,6 +173,8 @@ finish_encode([<<_/binary>>=B | Rest], Acc) ->
 finish_encode([Val | Rest], Acc) when is_integer(Val) ->
     Bin = list_to_binary(integer_to_list(Val)),
     finish_encode(Rest, [Bin | Acc]);
+finish_encode([{json, Json} | Rest], Acc) ->
+    finish_encode(Rest, [Json | Acc]);
 finish_encode([InvalidEjson | _], _) ->
     error({invalid_ejson, InvalidEjson});
 finish_encode(_, _) ->