Implement driver.Validator interface (go-sql-driver#1174)

Author: trongbq

Diff for: connection.go

@@ -642,3 +642,9 @@ func (mc *mysqlConn) ResetSession(ctx context.Context) error {
 	mc.reset = true
 	return nil
 }
+
+// IsValid implements driver.Validator interface
+// (From Go 1.15)
+func (mc *mysqlConn) IsValid() bool {
+	return !mc.closed.IsSet()
+}