Prevent open redirects when a malformed URL is passed to ?next=

Example: "/login?next=http:///google.com" (note 3rd slash)
Update to use (url_next.netloc or url_next.scheme) in the validate_redirect_url open redirect patch.
Merge pull request #318 from boydgreenfield/develop

Prevent open redirects when a malformed URL is passed to ?next=