Check to see if response has Content-Length before setting it.

Because of https://github.com/codysoyland/django-phased/issues/12, I had
to move PhasedRenderMiddleware to the bottom(ish) of my
`MIDDLEWARE_CLASSES`. This fixed the issue with the `MessageMiddleware`,
but caused new issues with other middleware I had that altered the
length of the response.

Ideally, these other middleware classes would check to see if
`Content-Length` was set, and if so, re-adjust the content length
header. Unfortunately, this doesn't seem to be the case, and it seems
smart for us to check to see if the header's set before adding it in
here, anyway.
Merge branch 'fix_content_length' into localwiki
-custom3