Enable users to provide null for either method or path as a wildcard.

I'm working on getting the swagger (http://swagger.wordnik.com/) API into an app for which I also using Spark 1.0. The code will need to enumerate all of the registered routes which are currently not exposed. This code change enables you to use the ```findTargetsForRequestedRoute``` method to do this by passing nulls where you don't care which value to match against. 

I've added some tests to demonstrate this.