(TK-33) Add support for location header munging on a proxied redirect

This PR adds an option to munge the location headers on redirect responses received by a proxy servlet. This option can be set in the options passed into the `add-proxy-route` function. It will be superseded by the `:follow-redirects` option if both are set at the same time.
I'm assuming that these tests work because the http client is configured to automatically follow redirects?  I think it would probably be better for us to disable that on the client and write the tests to check for the 30x header, and validate the munged Location header explicitly.  It's possible that we don't yet expose an option on the http client to disable redirects, and if that's the case, then it's probably worth adding that in now and then basing these tests off of it.  (We're 100% for sure going to need to support an option like that on the client in the future anyway.)