Fix for issue #1207

The documentation indicates that the second argument passed
to an asset_cache_buster method will be a string, but a File
is passed instead.

This poses the following problems:
1. A fatal error occurs if the file does not exist
2. The file descriptor is left open if the file exists

This commit addresses the issue by passing the real_path to
the provided asset_cache_buster method instead of a File instance