[6.x] Fix required_if boolean validation

[driesvints]

Re-send of #36967

This should fix some regressions for the changed behavior introduced in #36504. The PR now takes into account the boolean validation rule when combining rules like required_if. As reported in #36952 atm the validator will incorrectly return incorrect error messages when a boolean value of 0 is used. By first checking if the rules for the subject of required_if contains boolean we can safely know that it should be converted to a boolean. This doesn't breaks previous tests and all tests introduced by @jessarcher still pass.

I'd appreciate a second pair of eyes on this @jessarcher @timacdonald @SjorsO @taylorotwell.

Fixes #36952

[jessarcher]

Looks good to me. Sorry for the headache!

[driesvints]

Thanks @jessarcher. Don't worry about it 🙂

[aromka]

This now breaks nullable values:

'foo' => 'nullable|boolean',
'faz' => 'nullable|required_if:foo,false'

Previously if foo was missing from the request - it was ignoring this validation, now it treats null as false and fails the validation.
Setting 'faz' => 'nullable|required_unless:foo,true,null' also doesn't work and still fails validation.

(Using Laravel 8.x)

From documentation:

...you will often need to mark your "optional" request fields as nullable if you do not want the validator to consider null values as invalid

This is no longer correct since validator considers null values as false making the validation fail on an optional field.

[driesvints]

@aromka I'm looking into this now.

[driesvints]

@aromka PR made here: #37128

[driesvints]

@aromka would appreciate your review on that one.

[oprypkhantc]

@driesvints This is still a breaking change. Every piece of validation that used 0/1 instead of false/true in required_if just stopped working as expected.

[uuf6429]

I don't like posting "me too" replies, but I agree with @oprypkhantc, we also have this problem and it's fixed by changing 1 to true.