Fatal PHP errors if "Show the SQL query" is enabled for Views' previews

Problem/Motivation

When enabling "Show the SQL query" in Views' settings and a filter for a JSON value is added, a PHP error happens on each preview because filters are passed as array where the preview expects a string.

In general, the way filters are passed around in this module is a hack and against the API, but that would require a bigger fix. A quick fix is to serialize the filters array to become a string.