Fix issue #7: Resolved unicode decoding exception in simple_str_filter

[thehsansaeed]

Fixed issue https://github.com/youtube/spitfire/issues/7: simple_str_filter was throwing an exception for unicode placeholders. Updated the code to handle unicode placeholders correctly without raising an exception.

The issue was that the simple_str_filter function was throwing an exception for unicode placeholders. This problem occurred when trying to encode unicode characters with the default filter, resulting in a unicode decode exception.

The fix involved updating the code to handle unicode placeholders correctly without raising an exception.