[#319] Do not return payload formatting error on regular objects

Regular objects by definition have valid payload, so there should
be no errors.

Related to #302

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/core/object/fmt.go

@@ -158,7 +158,7 @@ func (v *FormatValidator) ValidateContent(o *Object) error {
 			}
 		}
 	default:
-		return errors.Errorf("(%T) unsupported object type %s", v, o.Type())
+		// ignore all other object types, they do not need payload formatting
 	}
 
 	return nil