[HAT] text-frontend VCL: set Content-Type if not set

Our Apache configuration on the application servers includes "DefaultType
application/octet-stream", which tells Apache to set a Content-Type header with
that value if a Content-Type header was not already present on the response.
Apache 2.4 drops support for this option on the ground that the correct
behavior (as mandated by RFC 1945 and 2616) is to not set a Content-Type header
if the content-type is not known.

We're fine with this in principle, but there's a bigger sense of "if it ain't broken".
It's very difficult to anticipate the impact of removing this behavior, because
there is no straightforward way to distinguish organic application/octet-stream
headers from default-set ones, and thus no straightforward way to know in which
cases (if any) we rely on this behavior.

So, do it Varnish instead. It's trivial to do in VCL, and it comes with the
added benefit of being easier to instrument if we decide we want to analyze
this in the future.

* https://issues.apache.org/bugzilla/show_bug.cgi?id=13986
* https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=440058

RT: 7700
Change-Id: I2574db03706e2bd5ba4f4903afd8f5a9fd83bb2a