Proposed fix for issues #1654. fix isSerializable for java jaxrs-cxf (#1656)

* Update model.mustache

fix serializable import for cxf-cdi

* Update pojo.mustache

add implements to the pojo if isSerializable is true

* Update model.mustache

add import statement to fix compile issue when {{#serializableModel}} is true

* Update pojo.mustache

remove extra space to prevent unecessary diffs in petstore examples: diff --git a/samples/server/petstore/jaxrs-cxf/src/gen/java/org/openapitools/model/*

modules/openapi-generator/src/main/resources/JavaJaxRS/cxf-cdi/model.mustache

@@ -2,6 +2,9 @@ package {{package}};
 
 {{#imports}}import {{import}};
 {{/imports}}
+{{#serializableModel}}
+import java.io.Serializable;
+{{/serializableModel}}
 {{#useBeanValidation}}
 import javax.validation.constraints.*;
 {{/useBeanValidation}}

modules/openapi-generator/src/main/resources/JavaJaxRS/cxf/model.mustache

@@ -2,6 +2,9 @@ package {{package}};
 
 {{#imports}}import {{import}};
 {{/imports}}
+{{#serializableModel}}
+import java.io.Serializable;
+{{/serializableModel}}
 {{#useBeanValidation}}
 import javax.validation.constraints.*;
 import javax.validation.Valid;

modules/openapi-generator/src/main/resources/JavaJaxRS/cxf/pojo.mustache

@@ -22,7 +22,7 @@ import com.fasterxml.jackson.annotation.JsonProperty;
  **/
 @ApiModel(description="{{{description}}}")
 {{/description}}
-public class {{classname}} {{#parent}}extends {{{parent}}}{{/parent}} {
+public class {{classname}} {{#parent}}extends {{{parent}}}{{/parent}}{{#serializableModel}} implements Serializable{{/serializableModel}} {
   {{#vars}}{{#isEnum}}{{^isContainer}}
 {{>enumClass}}{{/isContainer}}{{#isContainer}}{{#mostInnerItems}}
 {{>enumClass}}{{/mostInnerItems}}{{/isContainer}}{{/isEnum}}