Hi,

In maybeExpandNamedQuery the driver.Valuer interface is tested too late IMHO. It should be the first thing to test in this function.

If I have a map[string]x parameter that implements driver.Valuer interface, it is caught by https://github.com/go-gorp/gorp/blob/master/gorp.go#L216 test...

My use case is to have a set map[string]struct{} with a Value method that changes it to a SQL array of strings.

I can submit a PR, if you agree.