[JAVA] Bug generating HashMap class

Description

I am trying to generate a HashMap class, but no class is generated and a plain Java Map is used where the class ContextHeaders should be used. It is working properly in 2.3.1 with swagger 2 but not in 3.0.3 with swagger 3.0 (3.0.3 still correctly generates the swagger 2.0 contract)

In 2.3.1 it generates ContextHeaders.java and it's used in the RulesUser class:

  @JsonProperty("contextHeaders")
  private ContextHeaders contextHeaders = null;

But in 3.0.3 there is no class and in the RulesUser class it has:

  @JsonProperty("contextHeaders")

  private Map contextHeaders = null;

Swagger-codegen version

3.0.3

Swagger declaration file content or url

swagger.json
swagger-2.3.1.json

Command line used for generation

swagger-client-config.json
java -jar ./swagger-codegen.jar generate -c ./swagger-client-config.json -i ./swagger.json -l java -o generated-client/

Steps to reproduce

Run the swagger codegen 3.0.3 against the swagger.json file with the config swagger-client-config.json.

Start with the linked swagger.json, swagger-client-config.json, and the Java generation command to reproduce the output. Compare the generated RulesUser class and the missing ContextHeaders class with the Swagger 2.3.1 result; done means the OpenAPI 3 input generates and uses ContextHeaders instead of a plain Map.