Fix assert_not_select on body for text or html

[3v0k4]

Hey!

First of all, thank you so much for your work on this gem 🙏

While working on a project, I wrote the following test:

get foo_path
assert_dom "body", /bar/
make_bar_disappear_in_the_body
get foo_path
assert_not_dom "body", /bar/

The problem is that, with the current code, the last assertion always passes (even with "bar" in the body).

This PR fixes it.