Fixed SchemaPrinter so that it uses late static binding

[fabis94]

Making class methods protected isn't enough to be able to override them, because all of them call each other using self:: not static::

[coveralls]

Coverage increased (+0.001%) to 86.186% when pulling 4e29b00 on fabis94:master into 8795c03 on webonyx:master.

[spawnia]

That opens the door for easier extensibility, nice.

[fabis94]

@spawnia when can I expect this to be merged/published?

[spawnia]

We typically wait for @vladar to show up and merge. This is a small change which I do feel confident about including, but so far that has been our modus operandi.

[vladar]

Thank you!