[SPR 12363] org.springframework.ui.ModelMap.mergeAttributes(Map) makes inefficient use of keySet iterator instead of entrySet iterator

It is more efficient to use an iterator on the entrySet of the map, to avoid the Map.get(key) lookup.
Test is ModelMapTests.java
@Test
public void testMergeMapWithOverriding() throws Exception {
Map beans = new HashMap();
beans.put("one", new TestBean("one"));
beans.put("two", new TestBean("two"));
beans.put("three", new TestBean("three"));
ModelMap model = new ModelMap();
model.put("one", new TestBean("oneOld"));
model.mergeAttributes(beans);
assertEquals(3, model.size());
assertEquals("oneOld", ((TestBean) model.get("one")).getName());
}