toObject uses wrong transform for subdocs

[mtsr]

As can be seen in the minimal reproduction here, when a schema and its subdoc both have transforms defined, the top-level transform is used for the subdoc as well.

Expected output:

{ _id: 515aef8c2b98ec5125000001,
  second: 
   [ { _id: 515aef8c2b98ec5125000002, secondToObject: true },
     { _id: 515aef8c2b98ec5125000003, secondToObject: true } ],
  firstToObject: true }

Actual output:

{ _id: 515aef8c2b98ec5125000001,
  second: 
   [ { _id: 515aef8c2b98ec5125000002, firstToObject: true },
     { _id: 515aef8c2b98ec5125000003, firstToObject: true } ],
  firstToObject: true }

In the gist I also included a workaround, as all actual fixes to mongoose I tried broke a bunch of tests. In my opinion this problem is caused by Document.prototype.toObject actually changing the options object. If anything it should change a clone, but that breaks a bunch of tests right now.