Add unit test for Bug Report in #7. Fix it according to the new approach based on tryAdvance().

Author: fmcarvalho

src/main/java/org/jayield/advs/AdvancerList.java

@@ -20,29 +20,23 @@
 import org.jayield.Traverser;
 import org.jayield.Yield;
 
-import java.util.Iterator;
 import java.util.List;
+import java.util.Spliterator;
 
 public class AdvancerList<U> implements Advancer<U>, Traverser<U> {
-    private final List<U> data;
-    private final Iterator<U> current;
+    private final Spliterator<U> current;
 
     public AdvancerList(List<U> data) {
-        this.data = data;
-        this.current = data.iterator();
+        this.current = data.spliterator();
     }
 
     @Override
     public void traverse(Yield<? super U> yield) {
-        if(!current.hasNext())
-            throw new IllegalStateException("Traverser has already been operated on or closed!");
-        data.forEach(yield::ret);
+        current.forEachRemaining(yield::ret);
     }
 
     @Override
     public boolean tryAdvance(Yield<? super U> yield) {
-        if(!current.hasNext()) return false;
-        yield.ret(current.next());
-        return true;
+        return current.tryAdvance(yield::ret);
     }
 }

src/test/java/org/jayield/QueryTraverseTest.java

@@ -289,6 +289,28 @@ public void testReduce() {
         assertEquals(actual, expected);
     }
 
+   @Test
+    public void testFlatMapAndReduce() {
+        List<Query<String>> input = new ArrayList<>();
+        input.add(Query.of("a"));
+        input.add(Query.of("b"));
+        input.add(Query.of("c"));
+        String expected = "abc";
+        String actual = fromList(input).flatMap(s -> s).reduce((p, c) -> p + c).orElseThrow();
+        assertEquals(actual, expected);
+    }
+
+    @Test
+    public void testFromListFlatMapAndReduce() {
+        List<Query<String>> input = new ArrayList<>();
+        input.add(fromList(List.of("a")));
+        input.add(fromList(List.of("b")));
+        input.add(fromList(List.of("c")));
+        String expected = "abc";
+        String actual = fromList(input).flatMap(s -> s).reduce((p, c) -> p + c).orElseThrow();
+        assertEquals(actual, expected);
+    }
+
     @Test
     public void testReduceOnEmpty() {
         String[] input = {};