Move Authorization from NoCache to Cache

Despite what the RFC says, all of our current vendors *do* cache requests
that have Authorization headers and no explicit Cache-Control headers in the
response. They are not sure why - but we've lived with this behaviour for
some time now and are happy to accept it.
Allow Authorization requests to be cached in mocks

This is pretty much the same as 12bd408. Per the change to
TestCacheHeaderAuthorization in the preceeding commit. This short circuits
Varnish's default behaviour in:

https://github.com/varnish/Varnish-Cache/blob/varnish-3.0.5/bin/varnishd/default.vcl#L63-L66
Merge pull request #107 from alphagov/76768770-cached_authorization

[Fixes #76768770] Move Authorization from NoCache to Cache