[JAVA][HYSTRIXCOMMAND][JAVA8] Incorrect Code when generating java8 client code with hystrixcommand response wrapper

Description

when generating a java client using:
language = java
and options:
dateLibrary = java8
responseWrapper = HystrixCommand
interfaceOnly = true

the result won't compile. It generates a default implementation of the api interface methods that return a ResponseEntity although the method signature says it should return a HystrixCommand<ResponseEntity>

e.g. that's what is generated on my interface:

default com.netflix.hystrix.HystrixCommand<ResponseEntity<StoreResponse>> v1SessionPost(@ApiParam(value = "Transaction Identifier" ,required=true) @RequestHeader(value="transactionId", required=true) String transactionId,@ApiParam(value = "Client Identifier" ,required=true) @RequestHeader(value="clientId", required=true) String clientId,@ApiParam(value = "store_request" ,required=true )  @Valid @RequestBody StoreRequest body) {
        if(getObjectMapper().isPresent() && getAcceptHeader().isPresent()) {
            if (getAcceptHeader().get().contains("application/json")) {
                try {
                    return new ResponseEntity<>(getObjectMapper().get().readValue("{  \"payload\" : { },  \"lifespan\" : 3000,  \"expiry_date\" : \"2018-10-05 18:21:16.096\",  \"ack\" : 1,  \"session_id\" : \"507f191e810c19729de860ea\"}", StoreResponse.class), HttpStatus.NOT_IMPLEMENTED);
                } catch (IOException e) {
                    log.error("Couldn't serialize response for content type application/json", e);
                    return new ResponseEntity<>(HttpStatus.INTERNAL_SERVER_ERROR);
                }
            }
        } else {
            log.warn("ObjectMapper or HttpServletRequest not configured in default MethodsApi interface so no example is generated");
        }
        return new ResponseEntity<>(HttpStatus.NOT_IMPLEMENTED);
    }

Swagger-codegen version

I've tried 2.3.1, 2.3.2 and 2.4.8 they all have the bug

Swagger declaration file content or url

you can use any yaml even PetStore

Command line used for generation

I used the maven plugin with this configuration:

<plugin>
	<groupId>io.swagger</groupId>
	<artifactId>swagger-codegen-maven-plugin</artifactId>
	<version>2.3.2-assist-patch</version>
	<executions>
		<execution>
			<goals>
				<goal>generate</goal>
			</goals>
			<configuration>
				<inputSpec>${project.basedir}/src/main/resources/session-manager.yaml</inputSpec>
				<language>spring</language>
				<modelPackage>it.vodafone.dxl.session.model</modelPackage
                                 <apiPackage>it.vodafone.dxl.session.api</apiPackage>
				<configOptions>
					<sourceFolder>src/gen/java/main</sourceFolder>
					<dateLibrary>java8</dateLibrary>
					<hideGenerationTimestamp>true</hideGenerationTimestamp>
					<library>spring-cloud</library>
					<interfaceOnly>true</interfaceOnly>
					<responseWrapper>HystrixCommand</responseWrapper>
					<components>
						<component>models</component>
						<component>apis</component>
					</components>
				</configOptions>
			</configuration>
		</execution>
	</executions>
</plugin>

Steps to reproduce

Related issues/PRs

none that i know of

Suggest a fix/enhancement

The HystrixCommand wrapper should be added to the inners of the default interface methods implementation so code would compile

Start with the swagger-codegen Maven plugin configuration using language=spring, dateLibrary=java8, interfaceOnly=true, and responseWrapper=HystrixCommand, then inspect the generated Java API interface and its default methods. Reproduce the compilation failure and update the relevant generation template so the default implementation return values match the HystrixCommand-wrapped signatures.