Rest-Client-Reactive: void methods not allowed

[fwippe]

Describe the bug

Calling proxy endpoint methods with return type void fail with error message Response could not be mapped to type void.

Stacktrace:

javax.ws.rs.ProcessingException: Response could not be mapped to type void
	at org.jboss.resteasy.reactive.client.impl.ClientReaderInterceptorContextImpl.proceed(ClientReaderInterceptorContextImpl.java:63)
	at org.jboss.resteasy.reactive.client.impl.ClientSerialisers.invokeClientReader(ClientSerialisers.java:141)
	at org.jboss.resteasy.reactive.client.impl.RestClientRequestContext.readEntity(RestClientRequestContext.java:174)
	at org.jboss.resteasy.reactive.client.handlers.ClientResponseCompleteRestHandler.mapToResponse(ClientResponseCompleteRestHandler.java:30)
	at org.jboss.resteasy.reactive.client.handlers.ClientResponseCompleteRestHandler.handle(ClientResponseCompleteRestHandler.java:16)
	at org.jboss.resteasy.reactive.client.handlers.ClientResponseCompleteRestHandler.handle(ClientResponseCompleteRestHandler.java:12)
	at org.jboss.resteasy.reactive.common.core.AbstractResteasyReactiveContext.run(AbstractResteasyReactiveContext.java:141)
	at org.jboss.resteasy.reactive.client.impl.RestClientRequestContext$1.lambda$execute$0(RestClientRequestContext.java:268)
	at io.vertx.core.impl.AbstractContext.dispatch(AbstractContext.java:100)
	at io.vertx.core.impl.AbstractContext.dispatch(AbstractContext.java:63)
	at io.vertx.core.impl.EventLoopContext.lambda$runOnContext$0(EventLoopContext.java:38)
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:164)
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:469)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:500)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.base/java.lang.Thread.run(Thread.java:834)

Work-around: Change return type to String.

Expected behavior

Successful REST call

Actual behavior

Failing REST call

How to Reproduce?

@QuarkusTest
public class RestServiceIT {
    @Inject @RestClient
    UserService userService;

    @Test
    public void mustCreateUser() {
        userService.createUser(new UserService.UserData("Max", "Max", "test@example.com"));
    }

    @RegisterRestClient(baseUri = "https://jsonplaceholder.typicode.com")
    public interface UserService {
        @Path("/users")
        @POST
        void createUser(UserData user);

        class UserData {
            @JsonProperty
            final String name;

            @JsonProperty
            final String username;

            @JsonProperty
            final String email;

            public UserData(String name, String username, String email) {
                this.name = name;
                this.username = username;
                this.email = email;
            }
        }
    }
}

Output of uname -a or ver

Microsoft Windows [Version 10.0.19042.1415]

Output of java -version

openjdk version "11.0.3" 2019-04-16 OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.3+7) OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.3+7, mixed mode)

GraalVM version (if different from Java)

No response

Quarkus version or git rev

2.6.1.Final

Build tool (ie. output of mvnw --version or gradlew --version)

Apache Maven 3.8.2 (ea98e05a04480131370aa0c110b8c54cf726c06f)

Additional information

No response

[quarkus-bot]

/cc @michalszynkiewicz