(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 It will be superseded by the `:follow-redirects` option if both are set at the same time.
That would be fine.  If we go that route then I don't think we should have an enum value called 'default', though; I think it should be called "none" or something that more explicitly describes the behavior (assuming we're able to come up with a meaningful name).