fixed content assertions and expectations for non-page Capybara::Nodes

use the node found by prepare_args to actually match text
allow Capybara::Node::Simple as argument of Minitest asserts (as Capybara allows them so should Minitest)
add tests for text assertions
add tests for text expectations

I actually ran into this bug while after catching the latest version of `minitest-capybara` with a hasty `bundle update` after I upgraded to `rbenv`. The new version broke my e-mail tests which used `capybara-email`. During testing I discovered that Minitest assertions would not work for `Capybara::Node::Simple` so I added that too since it was in the same file.