Allow input type checkbox without need to set attribute "name" in version 2

[robertfausk]

Currently, PR #550 is only available in the 1.x branch.

@dunglas Would it be possible to merge it into main and create a new release for version 2?
Or do you need a new PR for this?

I need this change to get the tests of minkphp/driver-testsuite running in robertfausk/mink-panther-driver.

[robertfausk]

#526 also needs an upstream merge:)