Pr/FileUtils - slurp file content as characters

Update `FileUtils::read_glob_content` to not alter the IO layers on the file handle. Now `read_file_content` will slurp content as utf-8 characters, closing #302.

Avoid double encoding in `Handler::File` for mime type that match `/^text/` by ensuring static content is read as bytes and flag that the response has already been encoded.