connectionAttributes not unescaped

[zhangyangyu]

Issue description

Recently, connectionAttributes is supported! Currently, the value is used directly which means some special characters may lead to error, for example / which is special for this driver. If we pass %2f then it is not unescaped.

MySQL Connector/J uses a same configuration style as us. Its url parser is complex but seems finally it just decodes the whole value.

Example code

db, err := sql.Open("mysql", fmt.Sprintf("%s:%s@tcp(%s:4000)/test?tls=tidb&connectionAttributes=program_name:pingcap/serverless-test", user, password, host))

Error log

failed to connect databaseinvalid DSN: did you forget to escape a param value?

Configuration

Driver version (or git SHA):

Go version: run go version in your console

Server version: E.g. MySQL 5.6, MariaDB 10.0.20

Server OS: E.g. Debian 8.1 (Jessie), Windows 10