[5.6] Fix assertCookie() now cookies are unserialized by default

[derekmd]

With the security fix, EncryptCookies middleware now has $serialize = false out-of-the-box so cookie
tests now fail with app('encrypter')->decrypt($cookieValue, $unserialize = true) always called by assertCookie().

This breaks the method signature by adding another optional argument but today's changes also (necessarily) do so ¯\_(ツ)_/¯.

To avoid changing the method signature in 5.5 & 5.6, an alternate solution is to hardcode assertCookie() to false:

app('encrypter')->decrypt($cookieValue, false)

And if anyone wishes to keep:

protected static $serialize = true;

in their EncryptCookies middleware, they must add macros to TestReponse with their own assertion methods.

[derekmd]

It looks like Laravel Dusk's InteractsWithCookies trait will need some fixes as well.