False Validation on ConfigMap and Secret when name contains _

Hi,

What happens:
If you try to validate a ConfigMap or Secret file which has a name with _ (db_env f.e.) it returns true.
Later, if you try to apply it kubectl tells you that it's notr valid

What should happen:
It should fail because it doesn't have a valid file name.

I liked this a lot so I'll try to take a look, but I'll leave the issue just in case