Fix test I'd broken with change of reporting in %who.

IPython/frontend/tests/test_prefilterfrontend.py

@@ -169,7 +169,7 b' def test_magic():'
     f.input_buffer += '%who'
     f._on_enter()
     out_value = f.out.getvalue()
-    assert_equal(out_value, 'In\tOut\tget_ipython\t\n')
+    assert_equal(out_value, 'Interactive namespace is empty.\n')
 
 
 @isolate_ipython0