"isOfShape" should allow keys not to be specified if that key can be undefined

[RebeccaStevens]

import {
  isNumber,
  isOfShape,
  isOneOf,
  isUndefined
} from 'type-guards';

const isFoo = isOfShape({
  bar: isOneOf(isNumber, isUndefined),
  baz: isNumber
});

// This is ok.
console.log(isFoo({ bar: undefined, baz: 123 }));

// But this doesn't work.
console.log(isFoo({ baz: 123 }));
// Expect result: true
// Actual result: false

[lazarljubenovic]

Related: #8

Thanks for submitting the issue and PR. I'll check it out during the weekend.

[RebeccaStevens]

😄 Thanks for the awesome library. I'd love to help with getting this library to 1.0. Let me know if there is anything you want me to help with. I'll probably make more PRs for some of the open issues.