Add a param 'unescapeUser' to format dns like 'testdb%25dbuser:password@...'

[qindj]

Description

First ,the format : "testdb%dbuser:password.." is used in MySQL on Azure...

Recently I used some other projects such as Telegraf(https://github.com/influxdata/telegraf) which using this library to connect to MySQL.

These projects check dsn using url.Parse(), if I use "testdb%dbuser:password.." , error is "invalid URL escape %db". I tried to use a escaped value like "testdb%25dbuser:password...", error is "2001 user is not found from the service " because the username is wrong (with %25 not %)

So I make some changes to make it working.

I don't know this matter belongs to this library or the projects which using this library.

Correct me if I have any mistake.

Thanks!

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[julienschmidt]

There are several other things that can go horrible wrong when trying to parse a DSN with url.Parse.
Our DSNs are unfortunately not valid URLs, no will they ever be without a breaking change.

[julienschmidt]

FYI: https://godoc.org/github.com/go-sql-driver/mysql#ParseDSN can be used to validate a DSN