Support updates that filter on multiple discriminator keys with `$in`

**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**
Using updateOne() on discriminated models properties does not work. Here is a repro script.

**If the current behavior is a bug, please provide the steps to reproduce.**
```javascript
const testSchema = new mongoose.Schema(
{
title: { type: String, required: true },
subtitle: String,
kind: { type: String, required: true }
},
{ timestamps: true, discriminatorKey: 'kind' }
);

const Test = mongoose.model('Test', testSchema);

const testSchemaChild = new mongoose.Schema({
label: String
});

const TestChild = Test.discriminator('TestChild', testSchemaChild, 'testchild');

(async () => {
await mongoose.dropDatabase();

try {
await Test.create({
title: 'Title 1',
subtitle: 'subtitle 1',
label: 'label 1',
kind: 'testchild'
});
await Test.create({
title: 'Title 2',
subtitle: 'subtitle 2',
kind: 'testchild'
});

let test = await Test.find();
console.log(JSON.stringify(test, null, 4));
const doc = await Test.updateOne(
{ label: 'label 1' },
{ label: 'NEW LABEL' }
);
console.log(JSON.stringify(doc, null, 4));

test = await Test.find();
console.log(JSON.stringify(test, null, 4));
} catch (error) {
console.log(`${error}`);
}
console.log(`END`);
process.exit(0);
})();
```

**Output**

```json
[
{
"kind": "testchild",
"_id": "5ceb9e2c5449f12f0707231f",
"title": "Title 1",
"subtitle": "subtitle 1",
"label": "label 1",
"createdAt": "2019-05-27T08:22:04.478Z",
"updatedAt": "2019-05-27T08:22:04.478Z",
"__v": 0
},
{
"kind": "testchild",
"_id": "5ceb9e2c5449f12f07072320",
"title": "Title 2",
"subtitle": "subtitle 2",
"createdAt": "2019-05-27T08:22:04.498Z",
"updatedAt": "2019-05-27T08:22:04.498Z",
"__v": 0
}
]
{
"n": 1,
"nModified": 1,
"ok": 1
}
[
{
"kind": "testchild",
"_id": "5ceb9e2c5449f12f0707231f",
"title": "Title 1",
"subtitle": "subtitle 1",
"label": "label 1",
"createdAt": "2019-05-27T08:22:04.478Z",
"updatedAt": "2019-05-27T08:22:04.509Z",
"__v": 0
},
{
"kind": "testchild",
"_id": "5ceb9e2c5449f12f07072320",
"title": "Title 2",
"subtitle": "subtitle 2",
"createdAt": "2019-05-27T08:22:04.498Z",
"updatedAt": "2019-05-27T08:22:04.498Z",
"__v": 0
}
]
END
```

**What is the expected behavior?**
I expected it to update label to the new label. Note that if you change the update to e.g. this, it works:

```javascript
const doc = await Test.updateOne(
{ title: 'Title 1' },
{ title: 'NEW' }
);
```

**What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.**
Moongose 5.5.2
Mongodb 3.6.3
Node.js 10.13.0

Start with the provided JavaScript reproduction of Mongoose's Test.discriminator() and Test.updateOne() behavior, comparing the label filter with the working title filter. Trace how discriminator-key filters are handled during updateOne(), then add coverage for the reported multi-key/$in case and confirm that the matching document's label changes to the new value.

Written by the indexing model from the issue text.