ReportListFilter with false AtNull values

[nferch]

Hi,

If you create a ReportListFilter with a false ...AtNull attribute, the attribute is not passed to the API. For example, this does not work:

        filter := h1.ReportListFilter{
                BountyAwardedAtNull: false,
                Program:             []string{"spotify"},
        }

This is because go-querystring considers false values to be empty: https://github.com/google/go-querystring/blob/master/query/encode.go#L53

I worked around this by changing the bool values in ReportListFilter to pointer to bool, but this is pretty ugly especially since you can't create a literal pointer to false in Go.