[codegen][JAVA] Generated model with both properties and additionalProperties of differing types cannot be deserialized

Description

When a definition in Swagger contains both regular properties (with complex value types) and additionalProperties with type string, then the generated Java model class cannot deserialized by e.g. Jackson since the class extends HashMap<String,String> and it's complex values are being "set" as a String.

A better way of generating the class would probably be that the model class has a Map<String,String> field/member which additional properties are deserialized into.

Swagger-codegen version

2.3.1

Swagger declaration file content or url

Example swagger file where the generated Java class extends HashMap<String,String> which then breaks deserialization of the codes object:

swagger: '2.0'
info:
  version: 1.0.0
  title: Example
paths:
  /foo:
    get:
      responses:
        '200':
          description: OK
          schema:
            $ref: '#/definitions/ExampleResponse'
definitions:
  ExampleResponse:
    type: object
    additionalProperties:
      type: string
    properties:
      codes:
        type: object
        description: An object containing various codes
        example:
          foo: bar
          hello: world

Maven config used for generation

			<plugin>
				<groupId>io.swagger</groupId>
				<artifactId>swagger-codegen-maven-plugin</artifactId>
				<version>2.3.1</version>
				<configuration>
					<groupId>${project.groupId}</groupId>
					<language>java</language>
					<generateApis>false</generateApis>
					<library>jersey2</library>
					<configOptions>
						<dateLibrary>java8</dateLibrary>
						<useBeanValidation>true</useBeanValidation>
						<useJaxbAnnotations>true</useJaxbAnnotations>
					</configOptions>
				</configuration>
			</plugin>

Suggest a fix/enhancement

I've worked around this issue in my project by having a customized mustache file without using the parent mustache property. I'm not sure what side-effects that may have for other projects though.

Start by comparing the generated Java model behavior with the customized swagger-model/src/swagger-codegen-templates/pojo.mustache template linked in the report, focusing on the parent mustache property. Reproduce generation from the supplied Swagger definition and verify that Jackson can deserialize both the complex codes property and string additional properties without conflicting types.