Fix creation of invalid JSON when serializing functions or symbols (#299)

[MetalMichael]

Fixes #299

Basically JSON.stringify doesn't work for Functions and Symbols, and results in undefined, which is invalid JSON: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#description

So instead, when dealing with additionalProperties, make sure we aren't trying to serialize symbols or functions.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added (N/A)
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[13:26] (fix-serializing-undefined) fast-json-stringify $ yarn run test
yarn run v1.22.4
$ npm run test:lint && npm run test:unit && npm run test:typescript

> [email protected] test:lint repos/fast-json-stringify
> standard
> [email protected] test:unit repos/fast-json-stringify
> tap -J test/*.test.js test/**/*.test.js

 PASS  test/const.test.js 6 OK 173.52ms
 PASS  test/clean-cache.test.js 2 OK 67.577ms
 PASS  test/basic.test.js 67 OK 242.574ms
 PASS  test/array.test.js 24 OK 250.685ms
 PASS  test/anyof.test.js 25 OK 353.127ms
 PASS  test/any.test.js 25 OK 124.641ms
 PASS  test/allof.test.js 11 OK 39.659ms
 PASS  test/additionalProperties.test.js 18 OK 216.726ms
 PASS  test/debug-mode.test.js 8 OK 218.186ms
 PASS  test/inferType.test.js 15 OK 105.097ms
 PASS  test/date.test.js 40 OK 361.89ms
 PASS  test/defaults.test.js 12 OK 151.718ms
 PASS  test/long.test.js 8 OK 143.049ms
 PASS  test/integer.test.js 34 OK 136.064ms
 PASS  test/invalidSchema.test.js 2 OK 25.13ms
 PASS  test/if-then-else.test.js 10 OK 567.512ms
 PASS  test/nestedObjects.test.js 1 OK 16.34ms
 PASS  test/missing-values.test.js 7 OK 152.738ms
 PASS  test/nullable.test.js 8 OK 76.919ms
 PASS  test/patternProperties.test.js 7 OK 89.467ms
 PASS  test/regex.test.js 3 OK 90.434ms
 PASS  test/sanitize.test.js 3 OK 7.231ms
 PASS  test/sanitize2.test.js 1 OK 4.405ms
 PASS  test/ref.test.js 52 OK 496.434ms
 PASS  test/oneof.test.js 20 OK 683.684ms
 PASS  test/required.test.js 23 OK 61.754ms
 PASS  test/sanitize6.test.js 1 OK 4.081ms
 PASS  test/sanitize4.test.js 1 OK 6.357ms
 PASS  test/sanitize3.test.js 1 OK 6.06ms
 PASS  test/sanitize5.test.js 1 OK 8.566ms
 PASS  test/surrogate.test.js 8 OK 108.537ms
 PASS  test/toJSON.test.js 7 OK 117.36ms
 PASS  test/side-effect.test.js 7 OK 314.225ms
 PASS  test/unknownFormats.test.js 1 OK 19.633ms
 PASS  test/json-schema-test-suite/draft4.test.js 3 OK 48.62ms
 PASS  test/json-schema-test-suite/draft6.test.js 4 OK 104.46ms
 PASS  test/typesArray.test.js 27 OK 245.222ms
 PASS  test/json-schema-test-suite/draft7.test.js 4 OK 71.275ms

                         
  🌈 SUMMARY RESULTS 🌈  
                         

Suites:   38 passed, 38 of 38 completed
Asserts:  497 passed, of 497
Time:     5s

> [email protected] test:typescript repos/fast-json-stringify
> tsc --project ./test/types/tsconfig.json

✨  Done in 10.00s.

[13:27] (fix-serializing-undefined) fast-json-stringify $ yarn run benchmark
yarn run v1.22.4
$ node bench.js
FJS creation x 47,784 ops/sec ±5.00% (86 runs sampled)
CJS creation x 153,326 ops/sec ±1.56% (90 runs sampled)
JSON.stringify array x 4,545 ops/sec ±0.42% (93 runs sampled)
fast-json-stringify array x 6,045 ops/sec ±0.68% (91 runs sampled)
compile-json-stringify array x 5,961 ops/sec ±0.70% (90 runs sampled)
JSON.stringify long string x 11,318 ops/sec ±0.38% (97 runs sampled)
fast-json-stringify long string x 11,163 ops/sec ±0.70% (93 runs sampled)
compile-json-stringify long string x 11,347 ops/sec ±0.41% (95 runs sampled)
JSON.stringify short string x 8,854,949 ops/sec ±0.48% (94 runs sampled)
fast-json-stringify short string x 27,650,889 ops/sec ±2.32% (90 runs sampled)
compile-json-stringify short string x 28,937,678 ops/sec ±0.44% (91 runs sampled)
JSON.stringify obj x 2,080,972 ops/sec ±0.57% (89 runs sampled)
fast-json-stringify obj x 11,239,549 ops/sec ±0.96% (94 runs sampled)
compile-json-stringify obj x 14,636,055 ops/sec ±0.64% (94 runs sampled)
✨  Done in 76.33s.
[13:28] (fix-serializing-undefined) fast-json-stringify $ 

[jsumners]

LGTM

[mcollina]

lgtm