Support for 'secure' option in Rack::Test

Currently you can't read the ```secure``` flag of a cookie when using Rack::Test, even though there is a method in Rack that supports it. The changes here simply allow reading this flag with Rack::Test as well, and I also updated the tests.

It looks like the Webkit driver behaves slightly different than the others, in that it returns ```nil``` when the secure flag is ```false```. I've patched this to return ```false``` instead, so it's consistent with the other drivers.

All tests are passing.