If we pass non-ASCII strings to create_csv_response, it will call
unicode() on it, and fail with UnicodeDecodeError. So pass Unicode
strings, or ASCII-only bytestrings for the headers.
Fix error in instructor dashboard, data download section, when trying to download a csv with the issued certificates in a spanish (es-419) configured edx-platform deployment returns a UnicodeEncodeError: 'ascii' codec can't encode character u'\xf3' in position 16: ordinal not in range(128)
