res.send(200, 0) sends 'null' instead of 0 or other falsy values with application/json

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Bug Report

Restify Version

8.3.3

Node.js Version

11.15.0

Expected behaviour

The client should get 0 when res.contentType = 'application/json'; res.send(200, 0) is done and false when res.contentType = 'application/json'; res.send(200, false) is done.

Actual behaviour

The client gets null.

Repro case

server.get('/zero', function(req, res, next) {
  res.contentType = 'application/json';
  res.send(200, 0);
  next();
});

Cause

var data = body ? JSON.stringify(body) : 'null'; in formatJSON formatter returns null if any falsy object is in the body. Now 0 isn't a valid JSON object, but it is a valid JSON value and the parsers are able to parse it. Sending any other number actually send the number, not the null, so I think sending actual 0 would be consistent.

Are you willing and able to fix this?

Yes.

Start at the formatJSON formatter identified in the issue and reproduce the behavior with the /zero route using application/json. Check the response for 0 and false, then verify that other numeric values still work. Done means falsy JSON values are returned as their actual values rather than null.