AngularJS change: Add ability to give listeners the old collection in $watchCollection

[teropa]

Applying angular/angular.js#6736

[teropa]

Note to readers: The issue discussed in the book, where $watchCollection listeners get the new value twice in their listener function, and never get the old value, has been fixed in Angular.

The corresponding changes for the book are:

1. Change the test case introduced on page 74 ("works like a normal watch for non-collections") to not make any assertions about the old value. We'll deal with it later.
2. Change the test case introduced on page 95 ("does not consider any object with a length property an array") to the way its done on the newest version of the book. Previously it relied on oldValue and it'll break after this change.
3. Implement the old collection handling as described on pages 96-99.