Use proper header name

Follows up on quarkusio#26467 which seems to have introduced
flakyness

(cherry picked from commit a6c4511)

independent-projects/resteasy-reactive/server/vertx/src/main/java/org/jboss/resteasy/reactive/server/vertx/ResteasyReactiveOutputStream.java

@@ -6,12 +6,12 @@
 import io.vertx.core.Context;
 import io.vertx.core.Handler;
 import io.vertx.core.buffer.Buffer;
-import io.vertx.core.http.HttpHeaders;
 import io.vertx.core.http.HttpServerRequest;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InterruptedIOException;
 import java.io.OutputStream;
+import javax.ws.rs.core.HttpHeaders;
 import javax.ws.rs.core.MultivaluedMap;
 import org.jboss.logging.Logger;
 import org.jboss.resteasy.reactive.server.core.LazyResponse;