Add missing test for setting tenant_id to same val

This feature was added in
f4439e4,
but it looks like there wasn't an accompanying spec to cover the new
behavior.

Added the `require "yaml"` because I was getting a NameError; I guess
rails/all didn't cover that for some reason.

spec/active_record_helper.rb

@@ -1,5 +1,6 @@
 require 'rails/all'
 require 'database_cleaner'
+require 'yaml'
 
 dbconfig = YAML::load(IO.read(File.join(File.dirname(__FILE__), 'database.yml')))
 ActiveRecord::Base.logger = Logger.new(File.join(File.dirname(__FILE__), "debug.log"))

spec/acts_as_tenant/model_extensions_spec.rb

@@ -27,6 +27,15 @@
     it { expect {@project.account_id = @account.id + 1}.to raise_error }
   end
 
+  describe 'setting tenant_id to the same value should not error' do
+    before do
+      @account = Account.create!(:name => 'foo')
+      @project = @account.projects.create!(:name => 'bar')
+    end
+
+    it { expect {@project.account_id = @account.id}.not_to raise_error }
+  end
+
   describe 'tenant_id should be mutable, if not already set' do
     before do
       @account = Account.create!(:name => 'foo')