Change bulkUpdate to return the affected rows

In order to avoid selecting the same data after performing a bulk update, we noticed that the update query [already returns](https://github.com/sequelize/sequelize/blob/f96f2f6b252cb052c2fb8da8f7de6ea11cd10e5f/lib/dialects/abstract/query-generator.js#L206-L209) the affected rows.

With this PR, executing an update will now receive the actual affected rows instead of just the row count.

This is related to issue #921.