Check for presence in Spree::CreditCard#expiry method

The expiry method needs to check for presence before parsing. An empty card expiration field will raise the error "no implicit conversion of nil into String" on submission.

Fixes spree#3847
Fixes spree#3896

core/app/models/spree/credit_card.rb

@@ -17,8 +17,10 @@ class CreditCard < ActiveRecord::Base
     alias_attribute :brand, :cc_type
 
     def expiry=(expiry)
-      self[:month], self[:year] = expiry.split(" / ")
-      self[:year] = "20" + self[:year]
+      if expiry.present?
+        self[:month], self[:year] = expiry.split(" / ")
+        self[:year] = "20" + self[:year]
+      end
     end
 
     def number=(num)

core/spec/models/spree/credit_card_spec.rb

@@ -150,6 +150,25 @@ def stub_rails_env(environment)
     end
   end
 
+  # Regression test for #3847 & #3896
+  context "#expiry=" do
+    it "can set with a 2-digit month and year" do
+      credit_card.expiry = '04 / 14'
+      credit_card.month = '04'
+      credit_card.year = '2014'
+    end
+
+    it "can set with a 2-digit month and 4-digit year" do
+      credit_card.expiry = '04 / 2014'
+      credit_card.month = '04'
+      credit_card.year = '2014'
+    end
+
+    it "does not blow up when passed an empty string" do
+      lambda { credit_card.expiry = '' }.should_not raise_error
+    end
+  end
+
   context "#cc_type=" do
     it "converts between the different types" do
       credit_card.cc_type = 'mastercard'