Fix this.skip from spec with HTML reporter

[danielstjules]

From #1945 (comment) Fixes the following simple case for the HTML reporter:

describe('skipping', function() {
  it('amazes', function() {
    this.skip()
    assert(true)
  })
})

// expected: "amazes" test shows as pending within test results
// actual outcome: "TypeError: Cannot read property 'toString' of undefined"

Before PR:

After PR:

[mislav]

Thanks @danielstjules!