Fix serialization of nested objects

[florianreinhart]

This fixes the serialization of nested objects when they contain the same properties as the parent, but use additionalProperties or patternProperties. Fixes #79.

Benchmark before

JSON.stringify array x 4,626 ops/sec ±0.36% (95 runs sampled)
fast-json-stringify array x 8,244 ops/sec ±0.66% (90 runs sampled)
fast-json-stringify-uglified array x 8,290 ops/sec ±0.51% (88 runs sampled)
JSON.stringify long string x 17,645 ops/sec ±0.36% (96 runs sampled)
fast-json-stringify long string x 17,400 ops/sec ±0.95% (94 runs sampled)
fast-json-stringify-uglified long string x 17,371 ops/sec ±1.10% (93 runs sampled)
JSON.stringify short string x 5,758,694 ops/sec ±0.15% (94 runs sampled)
fast-json-stringify short string x 32,446,046 ops/sec ±0.35% (96 runs sampled)
fast-json-stringify-uglified short string x 32,126,762 ops/sec ±0.22% (92 runs sampled)
JSON.stringify obj x 2,165,489 ops/sec ±0.44% (95 runs sampled)
fast-json-stringify obj x 7,150,784 ops/sec ±0.47% (95 runs sampled)
fast-json-stringify-uglified obj x 7,092,883 ops/sec ±0.73% (89 runs sampled)

Benchmark after

JSON.stringify array x 4,628 ops/sec ±0.37% (94 runs sampled)
fast-json-stringify array x 8,244 ops/sec ±0.78% (88 runs sampled)
fast-json-stringify-uglified array x 8,132 ops/sec ±0.60% (88 runs sampled)
JSON.stringify long string x 17,656 ops/sec ±0.32% (94 runs sampled)
fast-json-stringify long string x 17,661 ops/sec ±0.16% (95 runs sampled)
fast-json-stringify-uglified long string x 17,615 ops/sec ±0.47% (95 runs sampled)
JSON.stringify short string x 5,748,825 ops/sec ±0.21% (98 runs sampled)
fast-json-stringify short string x 27,898,277 ops/sec ±0.58% (93 runs sampled)
fast-json-stringify-uglified short string x 33,354,991 ops/sec ±0.26% (93 runs sampled)
JSON.stringify obj x 2,115,439 ops/sec ±0.62% (94 runs sampled)
fast-json-stringify obj x 7,024,289 ops/sec ±0.43% (93 runs sampled)
fast-json-stringify-uglified obj x 7,155,966 ops/sec ±0.68% (93 runs sampled)

[jsumners]

LGTM

[mcollina]

LGTM

[delvedor]

LGTM

[themre]

will this be merged?

[mcollina]

LGTM

[mcollina]

Thanks!