added test that changes color at same position

src/test/java/no/uib/inf101/colorgrid/TestColorGrid.java

@@ -25,6 +25,10 @@ public class TestColorGrid {
     grid.set(new CellPosition(1, 2), Color.RED);
     assertEquals(Color.RED, grid.get(new CellPosition(1, 2)));
     assertEquals(null, grid.get(new CellPosition(2, 1)));
+
+    // Check that we can set a new value and retrieve it again
+    grid.set(new CellPosition(1, 2), Color.GREEN);
+    assertEquals(Color.GREEN, grid.get(new CellPosition(1, 2)));
   }
 
   @Test