td.matchers.contains blows up on sparse matches

[searls]

Just noticed this while writing docs:

matcher = td.matchers.contains({container: {size: 'S'}})

matcher.__matches({}) // blows up, can't check 'size' of undefined
matcher.__matches({container: {}}) // false does not blow up

We need to make sure that the contains matcher doing the deep object search fails fast when an intermediate property is absent