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As the `deps` for `django21` and `django22` were undefined, `tox` ended
up installing the latests version of Django in the environment instead.

if `request.META["HTTP_USER_AGENT"]` is not set the fallback value should an empty string instead of None
or else we get a `TypeError: argument of type 'NoneType' is not iterable` on `if 'WebKit' in user_agent:`

Now I raise a django builtin permission denied instead of writing a custom HttpResponse

otherwise the coverage env is missing these dependencies

This is not implemented using a `def head()` in the view, as the HEAD request
should return all headers that the GET request would send. Hence,
updated the server logic to provide a proper HEAD request.

Fix spelling error