Add req to defaultIndex args

[rreusser]

The API docs for defaultIndex state the format of a custom function as fn(params, req), so either I'm misinterpreting the docs or it's missing req so that the actual call is just fn(params). This PR adds the req argument and a corresponding test. Thanks for your time!

Notes:

• current invocation is here
• simple-html-index ignores the second argument, so hopefully shouldn't be a breaking change (though could certainly just skip passing it to that)

[mattdesl]

Nice catch! Thanks. 😄