Hi, I'm adding tests for capped collections in mongosync, and I found what may be a bug with modifying the maximum amount of documents a capped collection can store. To reproduce it, run something like:

db.createCollection( "cap", { capped: true, size: 10000000, max: 5 } )
db.cap.insertOne({"test":3})
db.cap.insertOne({"test":3})
db.cap.insertOne({"test":3})
db.cap.insertOne({"test":3})
db.cap.insertOne({"test":3})
db.runCommand({"collMod": "cap", "cappedSize": 100000000, "cappedMax":2})

//I'd expect there to be 2 documents in the collection, but there are still 5.
replset [direct: primary] test> db.cap.find()
[
  { _id: ObjectId("641b57cf4fec0a5858b5f8a0"), test: 3 },
  { _id: ObjectId("641b57d04fec0a5858b5f8a1"), test: 3 },
  { _id: ObjectId("641b57d14fec0a5858b5f8a2"), test: 3 },
  { _id: ObjectId("641b57d24fec0a5858b5f8a3"), test: 3 },
  { _id: ObjectId("641b57d24fec0a5858b5f8a4"), test: 3 }
]

//After inserting one more document, then it truncates to the correct max size
db.cap.insertOne({"test":4})
[
  { _id: ObjectId("641b57f14fec0a5858b5f8a5"), test: 3 },
  { _id: ObjectId("641b59564fec0a5858b5f8a7"), test: 4 }
]

Essentially changing a capped collections max number of documents to be smaller than the number of documents that it currently contains doesn't seem to remove documents, until a new insert is processed. Is this expected?