Only encode to ``utf8`` if content is unicode. - #312

Before this change,
when the ``compress`` management command is used
in conjunction with the ``COMPRESS_OFFLINE = True`` setting,
an error would occur if there were non-ASCII characters:

    Compressing... Error: An error occured during rendering /path/to/template/filename.html: 'ascii' codec can't decode byte 0x.. in position ####: ordinal not in range(128)