Image filter: configuration inheritance fixes.

The image_filter_jpeg_quality, image_filter_sharpen and "image_filter rotate"
were inherited incorrectly if a directive with variables was defined, and
then redefined to a literal value, i.e. in configurations like

    image_filter_jpeg_quality $arg_q;

    location / {
        image_filter_jpeg_quality 50;
    }

Patch by Ian Babrou, with minor changes.