Error due to change in pandoc AST

[bumper314]

The error:

Fatal error: Cannot use object of type stdClass as array in pandocfilters-php/pandocfilters.php on line 90

At some point pandoc changed the meta part of the AST. Here's how pandocfilters (python) fixed it, but I don't know how to achieve the same result in php: https://github.com/jgm/pandocfilters/commit/039dcc65bc10a14f9038a3671c795198bd50c15a