CSHARP-1515: Dispose of cursor in AsyncCursorEnumeratorAdapter.

Author: rstam

src/MongoDB.Driver/Sync/AsyncCursorEnumeratorAdapter.cs

@@ -42,14 +42,21 @@ public IEnumerator<TDocument> GetEnumerator()
                 throw new ObjectDisposedException(GetType().FullName);
             }
 
-            while (_cursor.MoveNext(_cancellationToken))
+            try
             {
-                var batch = _cursor.Current;
-                foreach (var document in batch)
+                while (_cursor.MoveNext(_cancellationToken))
                 {
-                    yield return document;
+                    var batch = _cursor.Current;
+                    foreach (var document in batch)
+                    {
+                        yield return document;
+                    }
                 }
             }
+            finally
+            {
+                Dispose();
+            }
         }
 
         public void Dispose()