Fix that MIGHT fix RSS for SOME people

Problem:
```pytb
Scanning posts..done!
.  render_site:output/categories/index.html
.  render_sources:output/blog/test-post.txt
.  render_archive:output/2013/index.html
.  render_tags:output/categories/tag1.xml
########################################
TaskError - taskid:render_tags:output/categories/tag1.xml
PythonAction Error
Traceback (most recent call last):
  File "/home/kwpolska/Desktop/nikola/lib/python2.7/site-packages/doit/action.py", line 326, in execute
    returned_value = self.py_callable(*self.args, **kwargs)
  File "/home/kwpolska/Desktop/nikola/lib/python2.7/site-packages/nikola/utils.py", line 330, in generic_rss_renderer
    rss_file.write(data)
  File "/home/kwpolska/Desktop/nikola/lib/python2.7/codecs.py", line 691, in write
    return self.writer.write(data)
  File "/home/kwpolska/Desktop/nikola/lib/python2.7/codecs.py", line 351, in write
    data, consumed = self.encode(object, self.errors)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 76: ordinal not in range(128)
```
Apparent solution: this patch.

**DISCLAIMER:** even though it worked for me, it may fail horribly, catch fire and eat your cat when you use it.  So, please do some testing before pulling it in.