fix: copy writable prototype props

[vkarpov15]

Hi,

Looks like #36 introduced a bug where prototype properties that don't have a setter won't get copied, even if they are writable. Example:

function X() {
  this.test = 'A';
}

Object.defineProperty(X.prototype, 'test', {
  configurable: false,
  writable: true,
  enumerable: true
});

// "X { test: 'A' }"
console.log(new X());
// "X {}", `test` prop omitted!
console.log(clone(new X()));

This issue came up in Automattic/mongoose#5896

Thanks for the great lib and happy holidays!

[olsonpm]

Just in case it's not clear, mdn defines descriptors as either being "data" or "accessor" descriptors. The current code assumes only the latter whereas both need to be accounted for.

A data descriptor also has the following optional keys:
value... writeable

An accessor descriptor also has the following optional keys:
get... set