goprotobuf: Fixes for Clone and Equal.

R=r
CC=golang-dev
http://codereview.appspot.com/6419071
diff --git a/proto/encode.go b/proto/encode.go
index d44b907..13a8123 100644
--- a/proto/encode.go
+++ b/proto/encode.go
@@ -263,7 +263,7 @@
 // All protocol buffer fields are nillable, but be careful.
 func isNil(v reflect.Value) bool {
 	switch v.Kind() {
-	case reflect.Map, reflect.Ptr, reflect.Slice:
+	case reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
 		return v.IsNil()
 	}
 	return false