Net::LDAP.new deprecation warning

When `encryption` parameter is passed as String to Net::LDAP.new

```
ldap_options = {
...
encryption: 'ssl',
}
ldap = GitHub::Ldap.new ldap_options
```

it produces the following [deprecation warning](https://github.com/ruby-ldap/ruby-net-ldap/blob/78e97ed69dc9ebbf06b04e7e70ceb046c23de75c/lib/net/ldap.rb#L622)
`Deprecation warning: please give :encryption option as a Hash to Net::LDAP.new`

At [present](https://github.com/ruby-ldap/ruby-net-ldap/blob/78e97ed69dc9ebbf06b04e7e70ceb046c23de75c/lib/net/ldap.rb#L483), the `encryption` value must be a Hash with parameters, which consists of two keys:
`method:` - `:simple_tls` or `:start_tls`
`tls_options:` - Hash of options for that method

从 GitHub::Ldap.new 入口开始，跟踪 ldap_options 如何传递到 Net::LDAP.new。将 String 形式与 lib/net/ldap.rb 第 483 行和第 622 行附近的内容进行比较，那里描述了接受的 Hash 结构和警告。当 LDAP 初始化不再产生此弃用警告时，即表示完成。

由索引模型根据 Issue 内容生成。