Fix auth switch failure on MySQL 8

[sile]

The current implementation fails to connect a MySQL 8 server if the authentication plugin of the user has been changed from the default caching_sha2_password (related issue: #779).

This PR addresses the problem.

Note that I'm not familiar with MySQL and not sure this patch is the correct solution to address this issue, so any advice is very welcomed.

Commands to reproduce the problem

Starts a MySQL server:

$ docker run -e MYSQL_DATABASE=db -e MYSQL_USER=foo \
    -e MYSQL_PASSWORD=password -e MYSQL_ROOT_PASSWORD=password -p 3306:3306 --rm mysql:8.0.22

Connects to the server with foo user (this will be succeeded):

use sqlx::Connection;

#[async_std::main]
async fn main() -> anyhow::Result<()> {
    let url = "mysql://foo:password@127.0.0.1/db";
    sqlx::mysql::MySqlConnection::connect(url).await?;
    Ok(())
}

Changes the auth plugin from the default to mysql_native_password:

use sqlx::Connection;

#[async_std::main]
async fn main() -> anyhow::Result<()> {
    let url = "mysql://root:password@127.0.0.1/db";
    let mut connection = sqlx::mysql::MySqlConnection::connect(url).await?;
    sqlx::query("ALTER USER 'foo' IDENTIFIED WITH mysql_native_password BY 'password'")
        .execute(&mut connection)
        .await?;
    Ok(())
}

Again, connects to the server with foo user (this will be failed):

use sqlx::Connection;

#[async_std::main]
async fn main() -> anyhow::Result<()> {
    let url = "mysql://foo:password@127.0.0.1/db";
    sqlx::mysql::MySqlConnection::connect(url).await?;
    Ok(())
}
// => Error: error returned from database: 1045 (28000): Access denied for user 'foo'@'172.17.0.1' (using password: YES)

Once the patch of this PR is applied, the above script can be executed without errors.

[mehcode]

I appreciate the fix. Thank you ❤️ . This is a very undocumented part of MySQL. Haha.

[mehcode]

Note that I'm not familiar with MySQL and not sure this patch is the correct solution to address this issue, so any advice is very welcomed.

I've done some extensive testing on next (slated to become the core of 0.6) and dropping the NUL works for all forms of auth that I can tell.

[sile]

Thank you for your review!