Always output Content-Length on mobile redirect

Return a zero Content-Length when 302ing from within Varnish. We have
evidence of at least one user agent that is broken enough to get
confused when the response doesn't have one (NTT DoCoMo) and it doesn't
hurt the rest.

Bug: 62245
Change-Id: I789c625d215de2f46a6f92647ada4263f35de8be