Remove on from schema reserved keys

[AbdelrahmanHafez]

We need to verify that people can use on as a path in their schemas.

[vkarpov15]

I confirmed that you can use on as a schema path. You get a warning that you can suppress using the supressReservedKeysWarning option.

As far as Mongoose itself is concerned, we don't need to warn about on as a reserved key. Documents support $on as an alternative, and Mongoose uses $on internally in the couple of places where we use documents as event emitters internally. My only concern is plugins that rely on .on().

@AbdelrahmanHafez what do you think about removing on from reserved keys in a minor release?

[AbdelrahmanHafez]

Nice.
Sounds good, there should be no risk in releasing this in a minor release. I'm anticipating that in a couple of years plugins will break due to those changes, but then again the fix is easy on the plugins' end. 👍