Fix integerish

[herndlm]

Closes #32

Fixed via 2 steps

• Using the correct expression instead of only is_numeric, see https://github.com/webmozarts/assert/blob/1.10.0/src/Assert.php#L99
• PHPStan retaining the types in that loosely equals comparison, see Retain left and right types in equal expression phpstan-src#1046

So this fix only works with the next PHPStan version.

[herndlm]

Extremely glad that this long-outstanding bug is finally fixed 🎉

[ondrejmirtes]

Makes complete sense :) Thank you.