simplify Event#url normalization.

Back at it with a simple little refactoring, here.

* Avoids magical AR lifecycle hooks
* Increases locality
* Only runs when url is set
* Replaces regex with semantically-identical and easier-to-grok #include? statement.

Well... nearly identical. Strings starting with "://" will match now, and they wouldn't before. Close enough. ;)
What's the reasoning behind changing this from a `Regex#match` to `String#include?`? Doesn't seem like it gets us much, especially if we lose matching capabilities.