(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 It will be superseded by the `:follow-redirects` option if both are set at the same time.
So, if I'm reading this correctly, the type of `response` here is `javax.servlet.HttpServletResponse`.

If that's true, and you're able to call `sendError` here, then it looks to me like there are a couple of other methods you could potentially call from within this function: `setHeader`, or `sendRedirect`.  If you can call either of those from here then it seems like you could get rid of your `filterResponseHeader` override and move all of that logic into this method, thus removing the need for the `nil` sentinel value that you're putting into the location header.  Did you already try those, and they won't work for some reason?