Make elements returned by `all` and `first` reloadable

Fixes #1410

All elements are considered reloadable. `#allow_reload!` may be removed in the future.

One of moments to discuss - currently behavior may be unexpected in the following case:
```ruby
el = all('li')[-1] # there are 4 'li' elements on the page
find('.remove_last_li').click
expect { el.text } not_to raise_error # I believe that will raise an error as that will search for third element, not the last one
```

But I don't know at the moment a good solution for it. The problem is that element of enumerable doesn't know the way using which it was got from that enumerable.