Adapt to the new Response API

Requires an Override of abstract headers() method.

Change-Id: If3e9724141f31d362a486aabdc181ad334fffc4d

src/main/java/com/googlesource/gerrit/plugins/eventseiffel/rest/EventCreationResponse.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/eventseiffel/rest/EventCreationResponse.java b/src/main/java/com/googlesource/gerrit/plugins/eventseiffel/rest/EventCreationResponse.java
index 8303940..61a766f 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/eventseiffel/rest/EventCreationResponse.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/eventseiffel/rest/EventCreationResponse.java
@@ -19,6 +19,7 @@
 import static com.googlesource.gerrit.plugins.eventseiffel.eiffel.dto.EiffelEventType.SCC;
 import static com.googlesource.gerrit.plugins.eventseiffel.eiffel.dto.EiffelEventType.SCS;
 
+import com.google.common.collect.ImmutableMultimap;
 import com.google.common.collect.Lists;
 import com.google.gerrit.extensions.restapi.CacheControl;
 import com.google.gerrit.extensions.restapi.Response;
@@ -78,6 +79,11 @@
   }
 
   @Override
+  public ImmutableMultimap<String, String> headers() {
+    return ImmutableMultimap.of();
+  }
+
+  @Override
   public String toString() {
     return "["
         + statusCode()