fix toJSON
fix toJSON

prometheusresearch/is-my-json-valid,kvnneff/is-my-json-valid,mafintosh/is-my-json-valid,ahdinosaur/is-my-json-valid,weo-edu/validate,simranjeet2589/is-my-json-valid,mafintosh/is-my-json-valid,cheeweep/is-my-json-valid,kgryte/is-my-json-valid,jonasfj/is-my-json-valid