Session reset (all cookie removal) using native Sahi calls

Before cookies were removed by setting their expiration to past date. This approach however had no effect, when cookie was set on different path, then the page where it's removal is attempted.

For example cookie set using `document.cookie = "some_cookie=ok; path=/";` was impossible to delete when user was on `/some/sub-page.html` and a code `document.cookie = "some_cookie=; expires=Thu, 01 Jan 1970 00:00:00 GMT";` was executed.

In this PR I've kept original cookie discovery code (plus added cookie name trim), but for cookie removal I'm using `_sahi._deleteCookie` call, which removes given cookie by name regardless of the path it was set on.