Allow omission of attributes if attributeOldValue or attributeFilter is present

[thompsongl]

Per the whatwg standard, attributes has no default value in MutationObserverInit (i.e., not false).
Also, attributes should be set to true if implicitly true (i.e., attributeOldValue or attributeFilter is specified).

The observe(target, options) method, when invoked, must run these steps:

1. If either options’s attributeOldValue or attributeFilter is present and options’s attributes is omitted, then set options’s attributes to true.

This PR will allow the omission of the attributes option if attributeOldValue or attributeFilter is specified.

---

Depending on your setup, you can work around this in the meantime by patching the observe method:

const MutationObserver = require('mutation-observer');
const _observe = MutationObserver.prototype.observe;
MutationObserver.prototype.observe = function observe(target, options) {
  const needsAttributes = options.hasOwnProperty('attributeOldValue') || options.hasOwnProperty('attributeFilter');
  if (needsAttributes && !options.hasOwnProperty('attributes')) {
    options.attributes = true;
  }
  Function.prototype.call(_observe, this, target, options);
};
Object.defineProperty(window, 'MutationObserver', { value: MutationObserver });

(h/t @chandlerprall)

[thompsongl]

Circling back to this to say that the above is only valid if IE11 support is not of concern.

The polyfill appears to implement an old(?) version of the spec in which attributes is required in the case described. It's likely better to handle the difference per project than to alter the polyfill.