Fix verification of the bypass cookie value

Hi,

I have just noticed a small regression since the last upgrade.
It's related to the functionality that allows us to bypass Varnish with a cookie.

This feature still works fine...
...until you want to delete the cookie. ^^

Indeed, the verification done with **Nexcessnet_Turpentine_Helper_Varnish:: isBypassEnabled()** is broken.
You try to use a type comparison with a boolean (the cookie value) and a string (the secret handshake). The result is inevitably always false and it's impossible to remove the cookie from the back-office.

I hope this will help.

Kind regards,
Alexandre Jardin