Fix use of trailing slashes in proxy

Fix use of trailing slashes in proxy
balancer and member URLs and add a little
explanation.

Current behavior AFAIK:

- Any path of a balancer URL inside <Proxy>
  is ignored. So lets not use e.g.trailing slashes
  there. The URL should for consistency always
  look like balancer://mycluster.
- The general comment about trailing slashes on the
  left and right of ProxyPass is also right for
  balancer use.
- Trailing slahes in BalancerMember in most cases
  lead to double slashes on the origin server.
  So lets drop them there and instead get the
  ProxyPass to the balancer right.
- Path components in member URLs are only
  needed, if different path prefixes are needed
  for members in the same balancer (unlikely
  but possible). If all members work with the
  same non-empty additional prefix, put it
  into the ProxyPass at the end of the balancer
  instead.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1421851 13f79535-47bb-0310-9956-ffa450edef68