[BrowserKit] Add `toArray` to `Response`

[fabpot]

Q	A
Feature PR	symfony/symfony#45515
PR author(s)	@HypeMC
Merged in	6.1

[javiereguiluz]

I've looked into this and I don't know where to add this new method. The most appropriate article would be components/browser_kit.rst but I don't know exactly where. Thanks.