Connect to DB whose name consists entirely of numerics

If the database name one wishes to connect to has only numeric, then the attempt fails with:

```
irb(main):008:0> client = Mysql2::Client.new(:host => '127.1.244.2', :database => 12345)                      
TypeError: can't convert Fixnum into String
        from /var/lib/openshift/524c65f6cd178a4609000113/.gem/gems/mysql2-0.3.13/lib/mysql2/client.rb:58:in `connect'
        from /var/lib/openshift/524c65f6cd178a4609000113/.gem/gems/mysql2-0.3.13/lib/mysql2/client.rb:58:in `initialize'
        from (irb):8:in `new'
        from (irb):8
        from /opt/rh/ruby193/root/usr/bin/irb:12:in `<main>'
```

A similar database works fine with the `pg` gem (configured via `config/database.yml` for a Rails app), so it might be a good idea to allow it.