fixes ascii error for tif imports

Windows users are getting ascii decoding error as mentioned in the GeoNode forum (https://groups.google.com/forum/#!topic/geonode-users/TvFIjqurUfg) when attempting to upload geotiffs.  The _get_content_type function returns a unicode, which fails to concatenate with strings in the CRLF.join(L) line.  Wrapping the function in the str function will typecast it appropriately.