Fix isOmitted to return true for omitted arguments

Closes gh-553

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/data/ArgumentValue.java

@@ -46,7 +46,7 @@
  */
 public final class ArgumentValue<T> {
 
-	private static final ArgumentValue<?> OMITTED = new ArgumentValue<>(null, false);
+	private static final ArgumentValue<?> OMITTED = new ArgumentValue<>(null, true);
 
 
 	@Nullable

spring-graphql/src/test/java/org/springframework/graphql/data/GraphQlArgumentBinderTests.java

@@ -230,8 +230,8 @@ void primaryConstructorWithOptionalArgumentBeanArgument() throws Exception {
 		itemBean = (PrimaryConstructorOptionalArgumentItemBean) result;
 
 		assertThat(itemBean).isNotNull();
-		assertThat(itemBean.getItem().isOmitted()).isFalse();
-		assertThat(itemBean.getName().isOmitted()).isFalse();
+		assertThat(itemBean.getItem().isOmitted()).isTrue();
+		assertThat(itemBean.getName().isPresent()).isFalse();
 	}
 
 	@Test