Adding getHasser (or getHazzer?) to Serializer.php

[fiboknacky]

With proto3, we now have some classes that contain a hasXXX() method. The googleads/google-ads-php repo needs to use a method similar to getGetter() to fetch the hasXXX() method name.

It'd be great if we can have such a method implemented right in Serializer (where getGetter and getSetter are written), so we don't need to write our own.