(#136) - Fix "changes limit and view instead of filter" against CouchDB 2.0

Do not assume incremental sequence numbers in the test.
This commit also adds a helper function to dig out specific items from
_changes (seems the simplest possible solution rather than importing
lodash or similar).
Merge pull request #3249 from willholley/136-changes-limit-and-view-instead-of-filter

(#136) - Fix "changes limit and view instead of filter" against CouchDB 2.0