Affects apps using rails >= 5.2

Rails 5.2 [dropped the
ActiveSupport::Hash#slice|https://github.com/rails/rails/commit/a524c955bfcce118da0ecb21affe28e2a102a54b] in favor of ruby's 2.5+ `Hash#slice`

This breaks the behaviour of `BSON::Document#slice` which relies on the call of `convert_key` method.

Example:

```
doc = mc.find(name: 'John').first

doc.class
=> BSON::Document

doc.slice(:name)
=> {}

doc.slice('name')
=>

```