Modify findOne example to check if result is null.

result can be null if there is no matching data.
Merge branch 'master' of github.com:SollmoStudio/beyond
Make collection.save method return a Future of the saved document on success.

Currently,  save method returns a Future of LastErrorMessage. But the updated
document is needed in most cases.

This patch makes save method return a Future of the saved document on success.
Merge branch 'master' of github.com:SollmoStudio/beyond