fix: don't assume options is set in belongsTo setter

lib/associations/belongs-to.js

@@ -121,6 +121,8 @@ module.exports = (function() {
     var association = this;
 
     instancePrototype[this.accessors.set] = function(associatedInstance, options) {
+      options = options || {};
+
       var value = associatedInstance;
       if (associatedInstance instanceof association.target.Instance) {
         value = associatedInstance[association.targetIdentifier];