Fix file downloader test

[philgyford]

The tests.core.test_utils.FileDownloaderTestCase.test_downloads_file generates this:

======================================================================
ERROR: test_downloads_file (tests.core.test_utils.FileDownloaderTestCase)
Streams a jpg, saves it to /tmp/, returns the path, calls _make_filename_for_download().
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Traceback (most recent call last):
  File "<string>", line 3, in wrapper
  File "/Users/phil/.pyenv/versions/3.5.1/lib/python3.5/unittest/mock.py", line 1157, in patched
    return func(*args, **keywargs)
  File "/Users/phil/Projects/personal/django-ditto/tests/core/test_utils.py", line 131, in test_downloads_file
    filepath = self.do_download()
  File "/Users/phil/Projects/personal/django-ditto/tests/core/test_utils.py", line 123, in do_download
    return filedownloader.download(self.url, ['image/jpeg'])
  File "/Users/phil/Projects/personal/django-ditto/ditto/core/utils/downloader.py", line 48, in download
    shutil.copyfileobj(r.raw, f)
  File "/Users/phil/Projects/personal/django-ditto/.tox/py35-django110/lib/python3.5/shutil.py", line 73, in copyfileobj
    buf = fsrc.read(length)
  File "/Users/phil/Projects/personal/django-ditto/.tox/py35-django110/lib/python3.5/site-packages/requests/packages/urllib3/response.py", line 380, in read
    data = self._fp.read(amt)
ValueError: I/O operation on closed file.

This happened when requests updated from v2.11.1 to v2.12.0.

No idea quite what's causing that and have spent too long trying to fix it.

http://stackoverflow.com/a/37283060/250962