(PUP-2738) Ensure block in user.password_is?

This ensures that a block is given by password_is? when calling
logon_user even though it doesn't need to do anything with it. We went this
route because it made more sense to require a block for this call versus
checking if a block was given in the logon_user method. If you call
logon_user without passing a block you cannot take the next step to later do
something as that user. So it would be a misleading if we allowed it without
requiring a block.
We have https://github.com/puppetlabs/puppet/blob/master/spec/unit/provider/user/windows_adsi_spec.rb#L102, though it couldn't hurt to add a test in user_spec, and remove the comment above.