Fix tests and example.

- Update testdata/underscore.go.html to match expected output after input change in b4219f1fb43b89dede6b8a2bc0c03f909cd599f7.
- Fix example verification to occur (see http://godoc.org/testing#hdr-Examples), and update expected output to match actual output.
- Copy latest version of `example_test.go` into `README.md` so they are in sync.