Fixed a bug that if you reset your session with open session, test suite crashes.

Related to:
https://github.com/jnicklas/capybara/issues/1253

Capybara #reset! should handle quite an ordinary case, where an alert might open up on the page.

Currently, if you try to use reset! and an alert opens on the page, then the capybara fails to handle it and crashes this and every other test that uses that same session