Cannot nest schema if setup with plugin

[wheresvic]

Hi,

We have a collection which is configured with the history plugin and it works fine. However if I also embed the schema in another collection which also has the history plugin setup, then mongoose-history throws an error on save

TypeError: Cannot read property 'name' of undefined
    at Document.<anonymous> (/xxx/app-name/node_modules/mongoose-history/lib/mongoose-history.js:31:79)

Basically, the save hook thinks that the subdocument also needs to be saved but it is part of a bigger document that has the history plugin setup.

One possible fix could be to check for this.collection before trying to actually save.