Fix non-base64 data URLs with % characters not followed by hex digits

When writing accumulated "non-special" characters, `slice_start` must be
updated as some later conditionals/patter matches don't update it like
the case when `%` is not followed by 2 hex digits.

This fixes servo#795

data-url/src/lib.rs

@@ -269,6 +269,7 @@ where
             // before this special byte
             if i > slice_start {
                 write_bytes(&bytes[slice_start..i])?;
+                slice_start = i;
             }
             // Then deal with the special byte.
             match byte {