Add coupon update

lib/stripe/coupon.rb

@@ -1,6 +1,7 @@
 module Stripe
   class Coupon < APIResource
     include Stripe::APIOperations::Create
+    include Stripe::APIOperations::Update
     include Stripe::APIOperations::Delete
     include Stripe::APIOperations::List
   end

test/stripe/coupon_test.rb

@@ -7,5 +7,14 @@ module Stripe
       c = Stripe::Coupon.create
       assert_equal "co_test_coupon", c.id
     end
+
+    should "coupons should be updateable" do
+      @mock.expects(:get).once.returns(test_response(test_coupon))
+      @mock.expects(:post).once.returns(test_response(test_coupon))
+      c = Stripe::Coupon.new("test_coupon")
+      c.refresh
+      c.metadata['foo'] = 'bar'
+      c.save
+    end
   end
-end
+end

test/test_helper.rb

@@ -201,7 +201,8 @@ def test_coupon(params={})
     :duration_in_months => 3,
     :percent_off => 25,
     :id => "co_test_coupon",
-    :object => "coupon"
+    :object => "coupon",
+    :metadata => {},
   }.merge(params)
 end