Default Object keys result in false positives

[ThomasRinsma]

Encodings are kept track of in a regular JavaScript Object. This causes problems when keys are requested, which exist in the default JavaScript Object prototype. For example:

> il = require("iconv-lite");
> il.encodingExists("constructor");
true

A fix would be to initialize such look-up objects as Object.create(null) (i.e. a null-prototype object).