maybeExpandNamedQuery & driver.Valuer conflict

[maxatome]

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.

[nelsam]

Sounds good to me. Might as well include the time.Time check early, too (probably in a type switch), and avoid any calls to reflect in those cases.