Fix HttpHeaderWriterWebFilterTests

Ensure setComplete() is subscribed to

web/src/test/java/org/springframework/security/web/server/header/HttpHeaderWriterWebFilterTests.java

@@ -67,7 +67,7 @@ public class HttpHeaderWriterWebFilterTests {
 
 		verify(writer, never()).writeHttpHeaders(any());
 
-		result.getExchange().getResponse().setComplete();
+		result.getExchange().getResponse().setComplete().block();
 
 		verify(writer).writeHttpHeaders(any());
 	}