Text spanning multiple invisible elements can be matched

The following step:

``` gherkin
Then I should see "A unicorn"
```

will match on an HTML:

``` html
<div>
  <span style="display: none">A</span> <span style="display: none">unicorn</span>
</div>
```

because even though none of the spans are visible, the outer `div` is, and it contains the specified text.
