fixes #6487 - fix getVirtual, set $nestedSchemaPath to full value of nestedSchemaPath

[ghost]

demonstrated in #6487

.$nestedSchemaPath wasn't getting set properly in getVirtual, resulting in the failure.

added a failing test, made it pass, all current tests pass:

mongoose>: npm test -- -g gh-6487

> [email protected] test /Users/lineus/dev/opc/mongoose
> mocha --exit test/*.test.js test/**/*.test.js "-g" "gh-6487"


 You're not testing shards!
 Please set the MONGOOSE_SHARD_TEST_URI env variable.
 e.g: `mongodb://localhost:27017/database
 Sharding must already be enabled on your database


  !

  0 passing (583ms)
  1 failing

  1) model: populate:
       github issues
         populates an array of objects
           handles embedded discriminator (gh-6487):

      AssertionError [ERR_ASSERTION]: 'Test name' === 'zyx'
      + expected - actual

      -Test name
      +zyx

      at test/model.populate.test.js:6532:18
      at Generator.next (<anonymous>)
      at onFulfilled (node_modules/co/index.js:65:19)
      at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:188:7)



npm ERR! Test failed.  See above for more details.
mongoose>: npm test -- -g gh-6487

> [email protected] test /Users/lineus/dev/opc/mongoose
> mocha --exit test/*.test.js test/**/*.test.js "-g" "gh-6487"


 You're not testing shards!
 Please set the MONGOOSE_SHARD_TEST_URI env variable.
 e.g: `mongodb://localhost:27017/database
 Sharding must already be enabled on your database


  ․

  1 passing (544ms)

mongoose>: npm test

> [email protected] test /Users/lineus/dev/opc/mongoose
> mocha --exit test/*.test.js test/**/*.test.js


 You're not testing shards!
 Please set the MONGOOSE_SHARD_TEST_URI env variable.
 e.g: `mongodb://localhost:27017/database
 Sharding must already be enabled on your database


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  1930 passing (27s)
  2 pending

mongoose>:

Output before change:

issues: ./6487_fails.js
[
  {
    "_id": "5b0556ec7eec4b3769381736",
    "nested": {
      "_id": "5b0556ec7eec4b3769381737",
      "events": [
        {
          "users": [
            1
          ],
          "_id": "5b0556ec7eec4b3769381739",
          "kind": "Clicked",
          "element": "#hero",
          "message": "hello"
        },
        {
          "_id": "5b0556ec7eec4b3769381738",
          "kind": "Purchased",
          "product": "action-figure-1",
          "message": "world"
        }
      ]
    },
    "__v": 0
  }
]
^C
issues:

Output after change:

issues: ./6487_fails.js
[
  {
    "_id": "5b0556dfbef4c13750320636",
    "nested": {
      "_id": "5b0556dfbef4c13750320637",
      "events": [
        {
          "users": [
            1
          ],
          "_id": "5b0556dfbef4c13750320639",
          "kind": "Clicked",
          "element": "#hero",
          "message": "hello",
          "users_$": [
            {
              "_id": "5b0556dfbef4c13750320635",
              "employeeId": 1,
              "name": "Test name",
              "__v": 0
            }
          ]
        },
        {
          "_id": "5b0556dfbef4c13750320638",
          "kind": "Purchased",
          "product": "action-figure-1",
          "message": "world",
          "users_$": []
        }
      ]
    },
    "__v": 0
  }
]
^C
issues:

[vkarpov15]

Good catch, thanks for finding this 🥇