Collection cursor has no field limits and will fetch entire documents

Meteor 2.7 version has replaced have "field" with "projection" key in _cursorDescription.options, which cause the error
```publish-counts: Collection cursor has no field limits and will fetch entire documents. consider specifying only required fields```
if ```countFromFieldLength``` method is used to get the count from the document field,

Start by tracing countFromFieldLength and the _cursorDescription.options handling that reads the cursor field limits. Compare Meteor 2.7's projection key with the current field lookup and verify that required-field limiting still prevents the entire document from being fetched while avoiding the reported error.

Written by the indexing model from the issue text.