[ADQL] Fix the way to test UDF features in FeatureSet

(i.e. `Object.equals` instead of `==`).

src/adql/parser/feature/FeatureSet.java

@@ -302,7 +302,7 @@ public class FeatureSet implements Iterable<LanguageFeature> {
 		if (type != null) {
 
 			// CASE: UDF
-			if (LanguageFeature.TYPE_UDF == type)
+			if (LanguageFeature.TYPE_UDF.equals(type))
 				done = anyUdfAllowed = true;
 
 			// OTHERWISE
@@ -403,7 +403,7 @@ public class FeatureSet implements Iterable<LanguageFeature> {
 		if (type != null) {
 
 			// CASE: UDF
-			if (LanguageFeature.TYPE_UDF == type)
+			if (LanguageFeature.TYPE_UDF.equals(type))
 				done = !(anyUdfAllowed = false);
 
 			// OTHERWISE
@@ -464,7 +464,7 @@ public class FeatureSet implements Iterable<LanguageFeature> {
 			return false;
 
 		// CASE: ANY UDF
-		if (anyUdfAllowed && LanguageFeature.TYPE_UDF == feature.type)
+		if (anyUdfAllowed && LanguageFeature.TYPE_UDF.equals(feature.type))
 			return true;
 
 		// OTHERWISE