Breaking change - safe: true in `flat_options` and `Backbone.Linear_Model.flatten` by default

I noticed, that in most cases more conveniently to work with simple arrays instead "objectified-arrays"
Therefore:

flat_options : {
    safe : true
}

Default behavior now.

For backward compatibility may have to set:

flat_options : {
    safe : false
}

If you expect, that arrays will be automatically objectified

Note: In static helper Backbone.Linear_Model.flatten safe : true by default too