Fix spanish character problem

[pando85]

This fix #45 in python 3.4 . I think it probably broke something in python2 but I don't know a best workaround for my problem right now.

[johnsensible]

I get this stack trace when I try to run the tests in Python 2.7 (Django 1.4):

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ERROR: test_xaccelredirect_header_containing_unicode (sendfile.tests.TestNginxBackend)
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Traceback (most recent call last):
  File "/home/john/Development/ace/venv/local/lib/python2.7/site-packages/sendfile/tests.py", line 128, in test_xaccelredirect_header_containing_unicode
    response = real_sendfile(HttpRequest(), filepath)
  File "/home/john/Development/ace/venv/local/lib/python2.7/site-packages/sendfile/__init__.py", line 68, in sendfile
    response = _sendfile(request, filename, mimetype=mimetype)
  File "/home/john/Development/ace/venv/local/lib/python2.7/site-packages/sendfile/backends/nginx.py", line 11, in sendfile
    response['X-Accel-Redirect'] = url
  File "/home/john/Development/ace/venv/local/lib/python2.7/site-packages/django/http/__init__.py", line 610, in __setitem__
    header, value = self._convert_to_ascii(header, value)
  File "/home/john/Development/ace/venv/local/lib/python2.7/site-packages/django/http/__init__.py", line 599, in _convert_to_ascii
    value = value.encode('us-ascii')
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 10: ordinal not in range(128), HTTP response headers must be in US-ASCII format

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Basically the trouble is that Django wants us-ascii data in the headers.

[pando85]

Yep, this was a silly solution that works for my problem but I wasn't sure that it could work in python 2.