modify convertValue() to fix issue 509 and add a test

src/main/java/com/beust/jcommander/JCommander.java

@@ -1375,11 +1375,11 @@ public Object convertValue(final Parameterized parameterized, Class type, String
         }
 
         IStringConverter<?> converter = null;
-        if (type.isAssignableFrom(List.class)) {
+        if (type.isAssignableFrom(List.class) || type.isAssignableFrom(Set.class)) {
             // If a list converter was specified, pass the value to it for direct conversion
             converter = tryInstantiateConverter(optionName, annotation.listConverter());
         }
-        if (type.isAssignableFrom(List.class) && converter == null) {
+        if ((type.isAssignableFrom(List.class) || type.isAssignableFrom(Set.class)) && converter == null) {
             // No list converter: use the single value converter and pass each parsed value to it individually
             final IParameterSplitter splitter = tryInstantiateConverter(null, annotation.splitter());
             converter = new DefaultListConverter(splitter, new IStringConverter() {

src/test/java/com/beust/jcommander/SetOfEnumTest.java

@@ -0,0 +1,37 @@
+package com.beust.jcommander;
+
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Set;
+
+public class SetOfEnumTest {
+
+    public enum Season{
+        SPRING,
+        SUMMER,
+        AUTUMN,
+        WINTER;
+    }
+
+    @Test
+    public void testParse()
+    {
+        class Args {
+            @Parameter(names = { "--season"}, description = "List of seasons separated by comma")
+            private Set<Season> seasons = new HashSet<>();
+        }
+        Args args = new Args();
+        JCommander.newBuilder()
+                .addObject(args)
+                .build()
+                .parse("--season", "SPRING");
+        Assert.assertEquals(Season.class, args.seasons.toArray()[0].getClass());
+    }
+
+    public static void main(String[] args) {
+        new SetOfEnumTest().testParse();
+    }
+}