allow setting belongs_to id to nil, even if nil is whiny

lib/machinist/active_record.rb

@@ -32,7 +32,7 @@ def self.assigned_attributes_without_associations(lathe)
       attributes = {}
       lathe.assigned_attributes.each_pair do |attribute, value|
         association = lathe.object.class.reflect_on_association(attribute)
-        if association && association.macro == :belongs_to
+        if association && association.macro == :belongs_to && !value.nil?
           attributes[association.primary_key_name.to_sym] = value.id
         else
           attributes[attribute] = value

spec/active_record_spec.rb

@@ -1,5 +1,6 @@
 require File.dirname(__FILE__) + '/spec_helper'
 require 'machinist/active_record'
+require 'active_support/whiny_nil'
 
 module MachinistActiveRecordSpecs
 
@@ -131,6 +132,16 @@ class Comment < ActiveRecord::Base
         comment[:post].should be_nil
         comment[:post_id].should_not be_nil
       end
+
+      describe "on a belongs_to association" do
+        it "should allow explicitly setting the association to nil" do
+          Comment.blueprint { post }
+          Comment.blueprint(:no_post) { post { nil } }
+          lambda {
+            @comment = Comment.plan(:no_post)
+          }.should_not raise_error
+        end
+      end
 
       describe "on a has_many association" do
         before do