Put timecop into safe mode and fix leaky Timecop.freeze

spec/models/spree/product_spec.rb

@@ -33,9 +33,10 @@ module Spree
       end
 
       it "defaults available_on to now" do
-        Timecop.freeze
-        product = Product.new
-        product.available_on.should == Time.zone.now
+        Timecop.freeze do
+          product = Product.new
+          product.available_on.should == Time.zone.now
+        end
       end
 
       describe "tax category" do