At least warn if you try to serialize a Boolean in a form body

[jnthn]

I just helped track down why a test looking like this failed:

        test-given '/get_excel_export', {
            test post(
                content-type => 'application/x-www-form-urlencoded',
                body => {
                    :datasetName('TestSingle'),
                    :language('de'),
                    :!withFilters
                }),
                status => 200;
        };

A urlencoded form cannot represent a boolean. It has no concept of anything but strings, really. A checkbox will not submit a boolean, but rather a value string or an empty string. We can certainly automatically stringify numbers as a convenience, but probably anything that's neither a string nor a number (excluding Bool) is probably not going to do anything useful. We should warn about this situation.