Fixed filtering by label (#250).

src/main/java/org/gitlab4j/api/models/MergeRequestFilter.java

@@ -300,7 +300,7 @@ public class MergeRequestFilter {
             .withParam("sort", sort)
             .withParam("milestone", milestone)
             .withParam("view", (simpleView != null && simpleView ? "simple" : null))
-            .withParam("labels", labels)
+            .withParam("labels", (labels != null? String.join(",", labels) : null))
             .withParam("created_after", createdAfter)
             .withParam("created_before", createdBefore)
             .withParam("updated_after", updatedAfter)