Add test for adding a custom response header for SSE resource method

extensions/resteasy-reactive/rest/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/customproviders/MultiResponseHeadersTest.java

@@ -0,0 +1,64 @@
+package io.quarkus.resteasy.reactive.server.test.customproviders;
+
+import static io.restassured.RestAssured.when;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.stringContainsInOrder;
+
+import jakarta.ws.rs.POST;
+import jakarta.ws.rs.Path;
+import jakarta.ws.rs.Produces;
+import jakarta.ws.rs.core.MediaType;
+
+import org.jboss.resteasy.reactive.RestMulti;
+import org.jboss.shrinkwrap.api.ShrinkWrap;
+import org.jboss.shrinkwrap.api.spec.JavaArchive;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import io.quarkus.test.QuarkusUnitTest;
+import io.smallrye.mutiny.Multi;
+
+public class MultiResponseHeadersTest {
+
+    @RegisterExtension
+    static QuarkusUnitTest test = new QuarkusUnitTest()
+            .setArchiveProducer(() -> ShrinkWrap.create(JavaArchive.class).addClasses(Resource.class));
+
+    @Test
+    public void headerAddedOnMulti() {
+        when()
+                .post("/multi")
+                .then()
+                .statusCode(200)
+                .header("Foo", equalTo("Bar"))
+                .body(stringContainsInOrder("hello", "world"));
+
+    }
+
+    @Test
+    public void headerAddedOnSseMulti() {
+        when()
+                .post("/sse-multi")
+                .then()
+                .statusCode(200)
+                .header("Foo", equalTo("Bar"))
+                .body(stringContainsInOrder("hello", "world"));
+
+    }
+
+    @Path("")
+    public static class Resource {
+        @POST
+        @Path("multi")
+        public Multi<String> multi() {
+            return RestMulti.fromMultiData(Multi.createFrom().items("hello", "world")).header("Foo", "Bar").build();
+        }
+
+        @POST
+        @Path("sse-multi")
+        @Produces(MediaType.SERVER_SENT_EVENTS)
+        public Multi<String> sseMulti() {
+            return RestMulti.fromMultiData(Multi.createFrom().items("hello", "world")).header("Foo", "Bar").build();
+        }
+    }
+}