fixes #6528 honor options when subpopulating

[lineus]

As demonstrated in #6528, when sub populating with options, the options aren't applied.

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

mongoose>: npm test -- -g 'gh-6528'

> mongoose@5.1.4-pre test /Users/lineus/dev/opc/mongoose
> mocha --exit test/*.test.js test/**/*.test.js "-g" "gh-6528"


 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 (609ms)
  1 failing

  1) model: populate:
       github issues
         populates an array of objects
           handles subpopulation with options (gh-6528):

      AssertionError: 2 === 1
      + expected - actual

      -2
      +1

      at test/model.populate.test.js:6507:18
      at next (native)
      at onFulfilled (node_modules/co/index.js:65:19)



npm ERR! Test failed.  See above for more details.
mongoose>: npm test -- -g 'gh-6528'

> mongoose@5.1.4-pre test /Users/lineus/dev/opc/mongoose
> mocha --exit test/*.test.js test/**/*.test.js "-g" "gh-6528"


 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 (600ms)

mongoose>: git status
On branch fix-6528
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

        modified:   lib/utils.js
        modified:   test/model.populate.test.js

no changes added to commit (use "git add" and/or "git commit -a")
mongoose>: lint
lib/utils.js
test/model.populate.test.js
mongoose>: npm test

> mongoose@5.1.4-pre 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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  1943 passing (36s)
  2 pending

mongoose>:

Output of 6528.js before change:

6528>: ./6528.js
{ AssertionError [ERR_ASSERTION]: 10 === 5
    at run (/Users/lineus/dev/Help/mongoose5/issues/6528/6528.js:38:10)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)
  generatedMessage: true,
  name: 'AssertionError [ERR_ASSERTION]',
  code: 'ERR_ASSERTION',
  actual: 10,
  expected: 5,
  operator: '===' }
^C
6528>:

Output of 6528.js after change:

6528>: ./6528.js
6528>:

[vkarpov15]

LGTM, nice 👍