correcting JsonSerialize as check

as is not required and returns Void by default.

Author: shawkins

modules/swagger-core/src/main/java/io/swagger/jackson/ModelResolver.java

@@ -311,7 +311,7 @@ public Model resolve(JavaType type, ModelConverterContext context, Iterator<Mode
         //If JsonSerialize(as=...) is specified then use that bean to figure out all the json-like bits
         JsonSerialize jasonSerialize = beanDesc.getClassAnnotations().get(JsonSerialize.class);
         if (jasonSerialize != null) {
-            if (jasonSerialize.as() != null) {
+            if (jasonSerialize.as() != Void.class) {
                 JavaType asType = _mapper.constructType(jasonSerialize.as());
                 beanDesc = _mapper.getSerializationConfig().introspect(asType);
             }