There's probably a difference in "can" vs. "should" here, but I couldn't find a huge amount of info on the issue itself.  The most relevant discussion is https://groups.google.com/forum/#!msg/nancy-web-framework/07G3oQF8XAM/Nkrn4FYRudMJ which resulted in a changeset that explicitly sets all of the response streams to UTF8 (https://github.com/kleinron/Nancy/commit/01808a1ae3894b165f64bf555fc5102272e3e98b).

I can't say for certain, and for that matter I'm not certain why Nancy leaves detection up to the browser, but it definitely seems safe to include the encoding (and necessary when using non-latin characters).