use default port if host does not have port

[bgaifullin]

If I do not specify port in DSN, connection fails with error:
failed: dial tcp: address localhost: missing port in address

Expected behaviour:
driver will connect by using default port: 3306

Configuration

Driver version (or git SHA):
sha:3955978caca48c1658a4bb7a9c6a0f084e326af3

Go version: run go version in your console
go version go1.9 darwin/amd64