Simplify UTF-8 handling in CSV.

Previously, we were using the UnicodeWriter class, but this performed
a lot of additional work we didn't need. Simplify, but keep the test from
https://github.com/birdsarah/django-spreadsheetresponsemixin/pull/1 which
is still the correct check for whether this works
Merge pull request #8 from birdsarah/simplify_utf8_handling

Simplify UTF-8 handling in CSV.