Add tests to more pgtypes

Int4, Int8, Date, Timestamptz

pgtype/date.go

@@ -50,10 +50,7 @@ func (src *Date) AssignTo(dst interface{}) error {
 			// if dst is a pointer to pointer, strip the pointer and try again
 			case reflect.Ptr:
 				if src.Status == Null {
-					if !el.IsNil() {
-						// if the destination pointer is not nil, nil it out
-						el.Set(reflect.Zero(el.Type()))
-					}
+					el.Set(reflect.Zero(el.Type()))
 					return nil
 				}
 				if el.IsNil() {