Added polymorphism test and virtual functions

[mondus]

Pull request fixes #2278 by making Writer class functions virtual to allow correct behaviour when PrettyWriter is cast to the base class.

Changes made

• [x] Unit Test which ensures that PrettyWriter maintains pretty writing when cast to Writer
• [x] Changes to Writer class functions to make them virtual

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}