Missing default Net value in default Config

[enrichman]

Issue description

I spent lot of times thinking about some weird networking issues with my containers but I've finally found which was the problem.

Despite the comment in the doc saying that the tcp protocol is the default for the Net field, the default Config struct from NewConfig is not setting it:

mysql/dsn.go

Lines 85 to 94 in 44553d6

	func NewConfig() *Config {
	cfg := &Config{
	Loc: time.UTC,
	MaxAllowedPacket: defaultMaxAllowedPacket,
	Logger: defaultLogger,
	AllowNativePasswords: true,
	CheckConnLiveness: true,
	}
	return cfg

Because of that when trying to set only the Addr the DSN will not be respected.

Example code

With only Addr set:

cfg := mysql.NewConfig()
cfg.Addr = "my-address"
fmt.Println(cfg.FormatDSN())

output:

/

With Addr and Net:

cfg = mysql.NewConfig()
cfg.Net = "tcp"
cfg.Addr = "my-address"
fmt.Println(cfg.FormatDSN())

output:

tcp(my-address)/

Expected behaviour

I was expecting to have the tcp(my-address)/ also in the first case.
If the fix to add the default tcp value to the default config in the NewConfig is ok I'll be happy to create a PR.