Should call 'toJSON' if it is defined on the object being stringified.

index.js

@@ -39,7 +39,10 @@ module.exports = function (obj, opts) {
                 throw new TypeError('Converting circular structure to JSON');
             }
             else seen.push(node);
-
+
+            if (node.toJSON && typeof node.toJSON === 'function') {
+              node = node.toJSON();
+            }            
             var keys = objectKeys(node).sort(cmp && cmp(node));
             var out = [];
             for (var i = 0; i < keys.length; i++) {

test/to-json.js

@@ -0,0 +1,8 @@
+var test = require('tape');
+var stringify = require('../');
+
+test('toJSON function', function (t) {
+    t.plan(1);
+    var obj = { one: 1, two: 2, toJSON: function() { return { one: 1 }; } };
+    t.equal(stringify(obj), '{"one":1}' );
+});