Fix dynamic multi-byte utf-8 files rebuilding.

Thor compares the new contents with the existing file by using
File.binread(destination) == new_content.
File.binread returns a string with ASCII_8BIT encoding, which will not
match the new_content if new_content contains multi-byte utf-8.

This patch simply encodes the new_content to ASCII_8BIT before passing
it to Thor.