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 so they are in sync.