Checked safe redirect paths

When your application is redirecting to another URL provided as a user-modifiable parameter (for example, a post login redirect in a custom login process), it is important to check it refers to a URL on this application to avoid providing an open redirect.

This function checks the URL, avoiding subtle mistakes, and providing a central place to update these checks should browser behaviour evolve.

function O.checkedSafeRedirectURLPath(rdr)

Returns the value of rdr if it is a safe redirect URL path, and null otherwise.

For example:

P.respond("GET,POST", "/do/something", [
  {parameter:"rdr", as:"string", optional:true}
], function(E, rdr) {
    rdr = O.checkedSafeRedirectURLPath(rdr);
    // Do some operation
    if(rdr) {