add failing test case for fields with underscores

Hi there,

currently fields that begin with underscore have to be put in quotation marks in order for the query to be parseable. It would be nice if it would work also without the quotation marks.

I've added two test cases, one which works where the field is escaped and one that currently results in an error.

Would be nice if someone could take a look at it and make that test pass. Or maybe point me into the right direction that I could do that myself.