[JAVA] etc.: generated code is incorrect for untyped schemas

Description

Generated code for schemas with no defined type is incorrect, because parsing seems to require a type property despite it being optional in the specification (where it defines values that can be of any type).

Swagger-codegen version

2.2.3, 2.3.0-SNAPSHOT

Swagger declaration file content or url

swagger: '2.0'
info:
  title: 'map of arbitrary values'
  version: '1'
produces: ["application/json"]
paths:
  /mapOfAnything:
    get:
      operationId: mapOfAnything
      responses:
        default:
          schema:
            description: 'map of arbitrary values'
            type: object
            additionalProperties:
              description: Any JSON type

Command line used for generation

./run-in-docker.sh generate -i swagger.yaml -l java

Steps to reproduce

Generate code from the above YAML. Observe that the response is treated as Object instead of Map<String, Object>, despite describing a map.

Related issues/PRs

None found.

Suggest a fix/enhancement

Treat untyped schemas as generically as possible.

Start with the supplied swagger.yaml and reproduce the issue using run-in-docker.sh generate -i swagger.yaml -l java. Inspect the Java generation path for the response schema and verify that the untyped additionalProperties case is represented as Map<String, Object> rather than Object.