Fix var reference error within Model.unload() and add regression tests.

[distracteddev]

Context

Previously, when calling Model.unload(), we were accidentally iterating over the properties of a CounterMap rather than iterating over the keys within the CounterMap.

Changes

• Updates Model.unload() to correctly iterate over the keys within the CounterMap's count property and pass the resulting document IDs into unfetchDoc, unsubscribeDoc, and _maybeUnloadDoc.
• Adds regression tests to the associated loading.js cases.

  Proof of failing tests before patch

  

[ericyhwang]

Good find, and thanks for writing tests to verify the issue's fixed!

This issue was introduction with the creation of CounterMap: https://github.com/derbyjs/racer/commit/0cb0bf2a2114f2c41432163b7824d791cfa4f34b#diff-e786030b6c8e8ddf44d387d7af5a48e3bb45c7898cb52f06c1452f2903d82ab5R117