COLLECTIONS-697: Ensure FixedSizeList respects underlying list's size

Formally document and test that if the size of the backing list of
FixedSizeList changes, then so does the FixedSizeList. This has been the
historical behavior. Letting it be part of the contract is reasonable,
expected, and worthwhile to note.

Fixes #55

src/main/java/org/apache/commons/collections4/list/FixedSizeList.java

@@ -31,6 +31,11 @@ import org.apache.commons.collections4.iterators.UnmodifiableIterator;
  * The add, remove, clear and retain operations are unsupported.
  * The set method is allowed (as it doesn't change the list size).
  * <p>
+ * NOTE:
+ * Modifying the decorated list directly would results in influencing the outcome
+ * of method calls on this object. For example, the bounds of this list would reflect
+ * a newly added object to the underlying list.
+ * <p>
  * This class is Serializable from Commons Collections 3.1.
  *
  * @param <E> the type of elements in this collection

src/test/java/org/apache/commons/collections4/list/FixedSizeListTest.java

@@ -16,6 +16,8 @@
  */
 package org.apache.commons.collections4.list;
 
+import org.junit.Assert;
+
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -66,4 +68,19 @@ public class FixedSizeListTest<E> extends AbstractListTest<E> {
 //        writeExternalFormToDisk((java.io.Serializable) getCollection(), "src/test/resources/data/test/FixedSizeList.fullCollection.version4.obj");
 //    }
 
+    public void testListAllowsMutationOfUnderlyingCollection() {
+
+        List<String> decoratedList = new ArrayList<>();
+        decoratedList.add("item 1");
+        decoratedList.add("item 2");
+        //
+        FixedSizeList<String> fixedSizeList = FixedSizeList.fixedSizeList(decoratedList);
+        int sizeBefore = fixedSizeList.size();
+        //
+        boolean changed = decoratedList.add("New Value");
+        Assert.assertTrue(changed);
+        //
+        Assert.assertEquals("Modifying an the underlying list is allowed",
+                sizeBefore + 1, fixedSizeList.size());
+    }
 }