support the same values for domain as rails' session stores usually do

This pull request addresses https://github.com/redis-store/redis-actionpack/issues/1 and https://github.com/redis-store/redis-store/issues/218, https://github.com/redis-store/redis-store/issues/108

I also needed to change the tests based on test in rails, so the options for the session store could be changed per test.