Simply interfaces slightly

request.go

@@ -1,16 +1,12 @@
 package kafka
 
-type requestAPI interface {
+type requestEncoder interface {
+	encoder
 	key() int16
 	version() int16
 	expectResponse() bool
 }
 
-type requestEncoder interface {
-	encoder
-	requestAPI
-}
-
 type request struct {
 	correlation_id int32
 	id             *string