Fixed issue where a field that was removed from a Record would still exist after a merge.

[mckaskle]

This could happen when a value changed to null. This fixes a bug in the following scenario:

1. Make a query where field x has value y. The result is cached.
2. Make the same query but bypass the cache. This time the field x has value null.
3. Make the same query again but allow fetching from the cache.

Expected Result: field x has value null.
Actual Result: field x still has value y.

The fix was to ensure that keys/fields that do not exist in a new record are removed from the old record during a merge.

[apollo-cla]

@mckaskle: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[mckaskle]

I'm having trouble resolving the conflicts so I'm going to start over and open a new PR (#68).