Use new static when creating new instances of Uri

[rbayliss]

Currently, it's impossible to extend the Uri class, since it creates new instances of self, instead of static. This behavior is also inconsistent, since in other places it does a clone, which will create a new instance of the extending class. This PR just changes the instance creation logic to always use new static.

[mtdowling]

These changes make the assumption that subclasses all use the same constructor signature; however, subclasses might override the constructor with a different signature. Because constructors are not required to follow the Liskov substitution principle (LSP), this is a concern and the reason I did not use static initially.

This behavior is also inconsistent, since in other places it does a clone, which will create a new instance of the extending class.

When using a clone, the constructor is not invoked -- only the __clone method. We do not use clone in the Uri class because the static methods you reference are used as factory methods (e.g., "named constructors") for the concrete Uri class.

[rbayliss]

Got it. Thanks for the clarification.