(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 It will be superseded by the `:follow-redirects` option if both are set at the same time.
Given that `:follow-redirects` would make `:munge-location-headers` moot, I still think it would be better to just have one option with three values.  For example, could have the name `:redirects` with values:

- `:default` - no special redirects behavior (same behavior as if the setting were not specified at all)
- `:munge-location-headers` - redirects returned to the proxy client but with the location header munged such that the client would follow redirects back through the proxy server
- `:follow-redirects-on-server` - any redirects returned from the target of the proxy server would be followed on the server (not returned to the proxy client to follow)