Set cookie to an empty value when called to remove, rather than 'a'

[G-Rath]

Currently remove sets the cookie value to 'a' which is confusing and means (among other things) calling get afterwards returns a value which isn't expected if you've just removed the cookie.

I can't find anything online indicating that it's a bad idea to set a cookie without a value (it seems that omitting the = is being debated, but foo=; should be completely valid).