Update puppet `instance?` methods to use `instance_of?` instead of `is_a?`

The Jira issue PUP-10105, solved an issue where puppet resource --to_yaml would emit puppet class tags by updating the way PScalarDataType#instance? checks whether an object is an instance of the said type. 

The problem was that some of the resource's fields would be ProcessOutput objects, which is a child class of ruby String, and the instance? method used `is_a?` to check whether the ProcessOutput object was an instance. In this case, due to the way `is_a?` works, the instance? method would return true, since the object is a child of String. 

To fix this issue, we updated the method to use `instance_of?` when checking Strings, so it would only return true for actual ruby Strings.

The next step would be to update the `instance?` methods for the rest of Data and RichData data types such as (Integer/Float/Fixnum/Bignum/Rational)