Monkeypatch webrat for Nokogiri compatibility

[petergoldstein]

This is an attempt to address the Webrat / Nokogiri compatibility issue discussed here. It monkeypatches Webrat to explicitly add the old default arguments to the invocation of to_xpath.

This PR picked an unrelated failure in Rails main specs because of this commit, which removes the body content from a redirect response. Devise has tests that check that body content, and those tests are now failing. I would suggest we remove those specs, and I'm happy to put up a PR to do so. Just let me know if such a PR is desired.

UPDATE: As the unrelated failure is now fixed in main, I've rebased this PR and it now exclusively addresses the Webrat issue.

cc: @carlosantoniodasilva, @flavorjones