Fetch with update: true behaves like fetch with add: true in 0.9.2

In 0.9.2., fetch with `add: true` would add in new models but not blow away old models. According to the 0.9.9 release notes, `update: true` is the equivalent option in 0.9.9. 
    If you were previously using collection.fetch({add: true}), use {update: true} now instead.

However, in the 0.9.9 release, `update: true` actually blows away old models. This is surprising behavior since the release notes state that it's the 0.9.9 equivalent of `add: true`. This commit sets the default options to update to not remove old models.


Fixes documentcloud/backbone#2008