Make it easier to match URL arguments with Mockito

Matching URL arguments with Mockito can be tricky. The issue is in the URL#equals() method which checks if the two URLs "reference equivalent hosts":

Two hosts are considered equivalent if both host names can be resolved into the same IP addresses; else if either host name can't be resolved, the host names must be equal without regard to case; or both host names equal to null.

This means that a line like this:

assertEquals(new URL("http://alice/test"), new URL("http://bob/test"));

might pass, depending on the environment where the test runs (i.e. depending on whether the URL domain / host is resolved or not).

In order to avoid this we need to use a custom URL argument matcher that compares the toString values. We could even have a generic "toStringEquals" matcher.