Docs and typos in minimongo.
Fields filtering should happen *before* transformations.

Since 'fields' options relate to mongo 'find' query, i.e. raw data in mongo, it
has nothing to do with transformed data.
Fields projection in Cursor.observeChanges works.

Now 'changed' callback is not even called if change didn't affect the result of
projection.

Note: diff.js distinguishes changes only on one granularity level.

Ex.: You have object { foo: { bar: 1, baz: 2 }, qux: "something" }, and you
observe a cursor things.find({}, { fields: { 'foo.bar': 1 } }).

If 'foo.baz' changes, you still get notified as the whole 'foo' field was
changed. However, if you change 'qux' only, callback will not fire.