Caching can cause problems

[btoc007]

Since in Javascript, map keys are always strings there are cases where unexpected cache hits can occur.

For example,

var _1 = base58.encode(202);
var _2 = base58.decode('202');
var _3 = base58.encode(202);

console.log(_1 === _3);

The result here is false where it should be true. Either caching can be removed or there needs to be two separate caches, one for encode and one for decode.