Fix 1bpp image decoding and add a test image (#252)

tests/decode_images.rs

@@ -216,6 +216,11 @@ fn issue_69() {
 //assert!(img_res.is_ok());
 //}
 
+#[test]
+fn test_tiled_gray_i1() {
+    test_image_sum_u8("tiled-gray-i1.tif", ColorType::Gray(1), 30531);
+}
+
 #[test]
 fn test_tiled_rgb_u8() {
     test_image_sum_u8("tiled-rgb-u8.tif", ColorType::RGB(8), 39528948);