Fix belongsTo where params

[mweibel]

Usually when adding to a foobar.getBla() a where param
like foobar.getBla({where: {x: "y"}}) it should extend
the existing where param ("id": id) with the additional
where param.
This didn't work for belongsTo associations until now.

[mweibel]

I don't think the build failed because of my changes, can you please restart it?

[janmeier]

Good catch - but I think an else case is missing here.

if (!Utils._.isUndefined(params.where)) {
  params.where = Utils._.extend({id:id}, params.where)
} else {
  params.where = { id : id }
}

[mweibel]

Oh yeah, will fix this in another commit.

[mweibel]

Travis broke again, In case you'd want to assure it works, restart the build please :) thanks