fix(filterFilter): comparator should return true if deep object key is undefined

Since the filtering of deep object predicates was fixed in https://github.com/angular/angular.js/commit/b4eed8ad94ce9719540462c1ee969dfd3c6b2355, an undefined  key in a deep object predicate did not return true un the comparator like it does for non-nested object predicates and strings. See this plunker:

http://plnkr.co/edit/uB7Mta?p=preview

The first commit in the pull request adds a check to the comparator to see if a key is undefined.
The second commit adds the same test for non-nested objects predicate, in line with the string filter test
The third commit fixes multiple style issues in filterSpec.js

I separated them to make the actual issue more visible

Closes #6222