serializableModel config option does not work for JAX-RS spec

[GrahamJohnson]

Description

JAX-RS spec serializableModel config option does not generate imports for java.io.Serializable, causing compile errors on implements Serializable.

Swagger-codegen version

2.2.3, master

Steps to reproduce

1. Add -DserializableModel=true to jaxrs-spec-petstore-server.sh
2. Run bin/jaxrs-spec-petstore-server.sh
3. Observer that generated model classes are generated with implements Serializable but no import for java.io.Serializable:

package io.swagger.model;

import io.swagger.model.Category;
import io.swagger.model.Tag;
import java.util.ArrayList;
import java.util.List;
import javax.validation.constraints.*;


import io.swagger.annotations.*;
import java.util.Objects;


public class Pet  implements Serializable {
// ...
}

Suggest a fix/enhancement

Add the following to JavaJaxRS/spec/model.mustache:

{{#serializableModel}}
import java.io.Serializable;
{{/serializableModel}}

This matches https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/JavaJaxRS/model.mustache#L13,L14,L15.

[GrahamJohnson]

PR: #6651