Invalidate cache on Rows.NextResultSet call

When reading multiple result sets from sqlx.Rows using StructScan,
some information obtained usCCCing costly reflect operations is
cached for performance optimisation.
However, different result sets in a single sqlx.Rows object might
have different structures, so cache should be invalidated and rebuild
after each NextResultSet call. Now this method is not overwritten,
so old cache interferes with new structures after first result set scan.

Relates #857

sqlx.go

@@ -5,7 +5,6 @@ import (
 	"database/sql/driver"
 	"errors"
 	"fmt"
-
 	"io/ioutil"
 	"path/filepath"
 	"reflect"
@@ -51,9 +50,9 @@ func mapper() *reflectx.Mapper {
 
 // isScannable takes the reflect.Type and the actual dest value and returns
 // whether or not it's Scannable.  Something is scannable if:
-//   * it is not a struct
-//   * it implements sql.Scanner
-//   * it has no exported fields
+//   - it is not a struct
+//   - it implements sql.Scanner
+//   - it has no exported fields
 func isScannable(t reflect.Type) bool {
 	if reflect.PtrTo(t).Implements(_scannerInterface) {
 		return true
@@ -633,6 +632,17 @@ func (r *Rows) StructScan(dest interface{}) error {
 	return r.Err()
 }
 
+// NextResultSet behaves like sql.NextResultSet, but cleans cache of previous result set.
+func (r *Rows) NextResultSet() bool {
+	if !r.Rows.NextResultSet() {
+		return false
+	}
+	r.started = false
+	r.fields = nil
+	r.values = nil
+	return true
+}
+
 // Connect to a database and verify with a ping.
 func Connect(driverName, dataSourceName string) (*DB, error) {
 	db, err := Open(driverName, dataSourceName)
@@ -884,9 +894,9 @@ func structOnlyError(t reflect.Type) error {
 // then each row must only have one column which can scan into that type.  This
 // allows you to do something like:
 //
-//    rows, _ := db.Query("select id from people;")
-//    var ids []int
-//    scanAll(rows, &ids, false)
+//	rows, _ := db.Query("select id from people;")
+//	var ids []int
+//	scanAll(rows, &ids, false)
 //
 // and ids will be a list of the id results.  I realize that this is a desirable
 // interface to expose to users, but for now it will only be exposed via changes