Override StripeObject#eql? and #hash to produce more optimistic e…

…quivalency

Overrides `#eql?` (hash equality) and `#hash` so that Stripe objects can
be used more easily as Hash keys and that certain other frameworks that
rely on these methods will have an easier time (e.g. RSpec's `change`,
see #687).

I think this might be a little controversial if we weren't already
overriding the `#==` implementation, but because we are, I think it
makes sense to extent it to these two methods as well.

lib/stripe/stripe_object.rb

@@ -96,6 +96,17 @@ def ==(other)
       other.is_a?(StripeObject) && @values == other.instance_variable_get(:@values)
     end
 
+    # Hash equality. As with ==, we consider two equivalent Stripe objects equal.
+    def eql?(other)
+      self == other
+    end
+
+    # As for equality, we hash to Stripe objects to the same value if they're
+    # equivalent objects.
+    def hash
+      @values.hash
+    end
+
     # Indicates whether or not the resource has been deleted on the server.
     # Note that some, but not all, resources can indicate whether they have
     # been deleted.

test/stripe/stripe_object_test.rb

@@ -117,6 +117,46 @@ class TestObject < Stripe::StripeObject; end
       end
     end
 
+    context "#eql?" do
+      should "produce the true for two equivalent Stripe objects" do
+        obj1 = Stripe::StripeObject.construct_from(id: 1, name: "Stripe")
+        obj2 = Stripe::StripeObject.construct_from(id: 1, name: "Stripe")
+        assert obj1.eql?(obj2)
+      end
+
+      should "produce false for non-equivalent Stripe objects" do
+        obj1 = Stripe::StripeObject.construct_from(id: 1, name: "Stripe")
+        obj2 = Stripe::StripeObject.construct_from(id: 2, name: "Stripe")
+        refute obj1.eql?(obj2)
+      end
+
+      should "produce false for different types" do
+        obj1 = Stripe::StripeObject.construct_from(id: 1, name: "Stripe")
+        obj2 = 7
+        refute obj1.eql?(obj2)
+      end
+    end
+
+    context "#hash" do
+      should "produce the same hash for two equivalent Stripe objects" do
+        obj1 = Stripe::StripeObject.construct_from(id: 1, name: "Stripe")
+        obj2 = Stripe::StripeObject.construct_from(id: 1, name: "Stripe")
+        assert_equal obj1.hash, obj2.hash
+      end
+
+      should "produce different hashes for non-equivalent Stripe objects" do
+        obj1 = Stripe::StripeObject.construct_from(id: 1, name: "Stripe")
+        obj2 = Stripe::StripeObject.construct_from(id: 2, name: "Stripe")
+        refute_equal obj1.hash, obj2.hash
+      end
+
+      should "produce different hashes for different types" do
+        obj1 = Stripe::StripeObject.construct_from(id: 1, name: "Stripe")
+        obj2 = 7
+        refute_equal obj1.hash, obj2.hash
+      end
+    end
+
     context "#to_hash" do
       should "skip calling to_hash on nil" do
         module NilWithToHash