#1312 Hotfix for custom SQL filter (state: open)

Session access inside a custom SQL filter e.g. {{param::session?name=mm_blog_post_id&default=0}}

Throws an exception like this: Using $this when not in object context

Unlike Contao\Input Contao\Session has to be instantiated by getInstance()

@binron pls check Travis https://travis-ci.org/MetaModels/core/builds/532345765?utm_source=github_status&utm_medium=notification and adapt your files