Use object with null prototype for various app properties

[EvanHahn]

See discussion here.

app.cache, app.engines, and app.settings are now created with Object.create(null) instead of {}.

This also adds a test that app.locals is created the same way.

[EvanHahn]

This has been open for over a year. No rush from me, but let me know if there's anything I can do to move this along.