fix mocking.URL to not panic(nil)

This change makes the panic look like the following:

    panic: (tigertonic.NotFoundHandler) (0x4a6f20,0x64f400) [recovered]

As a side note, panic(nil) causes the test to pass! This isfixed in go1.3,
https://code.google.com/p/go/issues/detail?id=6546
Merge pull request #57 from rcrowley/mocking-panic-nil

fix mocking.URL to not panic(nil)