(PE-5859) Keep entire request URI matching regex

When wrap-proxy is called with a regex, keep the entire
portion of the request URI matching the regex rather than
replacing it in the rewritten URI.
@camlow325 This is definitely an issue that can occur, but the only URI path concatenation functions I can find are in Jetty and Apache, and I don't know if ring-middleware should have a dependency on those just for a URI Utility function (although I can add a Jetty dependency if others think it's a good idea).

Alternatively, I could strip out any trailing slashes in `remote-uri-base` before I turn it into a URI. That should avoid this problem.