Added SGI test for saving an image in an unsupported mode

Tests/test_file_sgi.py

@@ -55,6 +55,12 @@ class TestFileSgi(PillowTestCase):
         for mode in ('L', 'RGB', 'RGBA'):
             roundtrip(hopper(mode))
 
+    def test_unsupported_mode(self):
+        im = hopper('LA')
+        out = self.tempfile('temp.sgi')
+
+        self.assertRaises(ValueError, lambda: im.save(out, format='sgi'))
+
 
 if __name__ == '__main__':
     unittest.main()