call toJSON method on object types

[jsardev]

Not sure if this is the way to go, but I didn't have any other idea how to accomplish the behavior. If you have any suggestions how to do this differently please share your idea!

Node 10.9.0

FJS creation x 7,423 ops/sec ±2.95% (92 runs sampled)
JSON.stringify array x 4,588 ops/sec ±1.22% (93 runs sampled)
fast-json-stringify array x 7,248 ops/sec ±0.40% (92 runs sampled)
fast-json-stringify-uglified array x 7,262 ops/sec ±0.44% (92 runs sampled)
JSON.stringify long string x 17,338 ops/sec ±0.26% (95 runs sampled)
fast-json-stringify long string x 17,267 ops/sec ±0.37% (94 runs sampled)
fast-json-stringify-uglified long string x 17,298 ops/sec ±0.34% (95 runs sampled)
JSON.stringify short string x 4,879,163 ops/sec ±0.29% (93 runs sampled)
fast-json-stringify short string x 33,254,002 ops/sec ±0.89% (91 runs sampled)
fast-json-stringify-uglified short string x 34,301,110 ops/sec ±0.80% (92 runs sampled)
JSON.stringify obj x 1,876,687 ops/sec ±1.69% (83 runs sampled)
fast-json-stringify obj x 6,803,734 ops/sec ±1.29% (88 runs sampled)
fast-json-stringify-uglified obj x 6,510,238 ops/sec ±2.29% (79 runs sampled)

Resolves #118

[mcollina]

LGTM