Fix throws `TypeError` when passing an arrow function

[wmzy]

Codes like this:

    should.throws(
      () => {
        throw new Error();
      },
      () => true
    );

won't work:

TypeError: Function has non-object prototype 'undefined' in instanceof check

[btd]

should.throws follow assert.throws behavior only. Your use case is not supported by assert.throws as i see.

[wmzy]

assert.throws support this. See the "Custom error validation" example.