FormatDSN does not set default parameters correctly

[SeanBenoit]

Issue description

Config.FormatDSN() will override the default values of AllowNativePasswords and MaxAllowedPackets specified in README.md with zero values (false instead of true and 0 instead of 4194304 respectively). This means that using it to generate the DSN to be passed to Open() will cause behaviour that does not match the behaviour specified in this package's documentation.

Example code

package main

import (
	"fmt"
	gomysql "github.com/Go-SQL-Driver/MySQL"
)

func main() {
	config := gomysql.Config{
		User:   "user",
		Passwd: "password",
		Net:    "tcp",
		Addr:   "hostname:Port",
		DBName: "dbname",
	}

	fmt.Println(config.FormatDSN())
	// Expected output: user:password@tcp(hostname:Port)/dbname
	// Actual output: user:password@tcp(hostname:Port)/dbname?allowNativePasswords=false&maxAllowedPacket=0
}

Configuration

Driver version (or git SHA): commit 7413002

Go version: go version go1.10.2 linux/amd64

Server version: N/A

Server OS: N/A

[julienschmidt]

The respective godoc clearly states that NewConfig() should be used to set defaults if a new config is created instead of parsing it from a DSN string. Otherwise you'll get zero values - that is false and 0.