How to insert document field value as ISODate with Ruby MongoDB driver?

I couldn't find any documentation on this but if you look at the official examples, you'll see this:

result = client[:restaurants].insert_one({  #...  grades: [    {      date: DateTime.strptime('2014-10-01', '%Y-%m-%d'),      grade: 'A',      score: 11    },    #...  ]  #...})

That would suggest that you can use simple DateTime instances to insert times into MongoDB. So what happens if we try that? Well:

irb>  mongo[:pancakes].insert_one(:kind => 'blueberry', :created_at => DateTime.now)

and then in MongoDB:

> db.pancakes.find(){ "_id" : ..., "kind" : "blueberry", "created_at" : ISODate("2016-05-15T17:44:12.096Z") }

The ISODate that we want is there.

Then if we pretend we're in Rails:

irb> require 'active_support/all' # To get to_datetimeirb> mongo[:pancakes].insert_one(:kind => 'banana', :created_at => '2016-05-15T06:11:42.235Z'.to_datetime)

we get this inside MongoDB:

> db.pancakes.find(){ "_id" : ObjectId("5738b56cf638ccf407c71ef5"), "kind" : "blueberry", "created_at" : ISODate("2016-05-15T17:44:12.096Z") }{ "_id" : ObjectId("5738b74ef638ccf4da4c2675"), "kind" : "banana", "created_at" : ISODate("2016-05-15T06:11:42.235Z") }

ISODates again.

I'm using version 2.2.5 of the official Ruby driver here.