Fix prototype pollution vulnerability

Closes #76

pointer.ts

@@ -69,6 +69,9 @@ export class Pointer {
     for (let i = 1, l = this.tokens.length; i < l; i++) {
       parent = value
       key = this.tokens[i]
+      if (key == '__proto__' || key == 'constructor' || key == 'prototype') {
+        continue
+      }
       // not sure if this the best way to handle non-existant paths...
       value = (parent || {})[key]
     }