Add question mark helpers (e.g. #paid?) for boolean object values

This patch adds question marks helpers (e.g. #paid?) for any values in a
StripeObject that are a boolean. This is fairly idiomatic Ruby in that
it behaves similarly to other libraries like ActiveRecord.

Note that a caveat here is that nullable booleans will not get a helper
added for them if their current value is null. For this reason, we
should eventually prefer to derive these methods from some sort of
programmatic API manifest.

Replaces #257 and #274.

lib/stripe/stripe_object.rb

@@ -47,7 +47,7 @@ module Stripe
 
       instance_eval do
         remove_accessors(removed)
-        add_accessors(added)
+        add_accessors(added, values)
       end
 
       removed.each do |k|
@@ -216,7 +216,7 @@ module Stripe
       end
     end
 
-    def add_accessors(keys)
+    def add_accessors(keys, values)
       metaclass.instance_eval do
         keys.each do |k|
           next if @@permanent_attributes.include?(k)
@@ -232,6 +232,11 @@ module Stripe
             @values[k] = v
             @unsaved_values.add(k)
           end
+
+          if [FalseClass, TrueClass].include?(values[k].class)
+            k_bool = :"#{k}?"
+            define_method(k_bool) { @values[k] }
+          end
         end
       end
     end
@@ -240,7 +245,10 @@ module Stripe
       # TODO: only allow setting in updateable classes.
       if name.to_s.end_with?('=')
         attr = name.to_s[0...-1].to_sym
-        add_accessors([attr])
+
+        # the second argument is only required when adding boolean accessors
+        add_accessors([attr], {})
+
         begin
           mth = method(name)
         rescue NameError

test/stripe/stripe_object_test.rb

@@ -31,5 +31,12 @@ module Stripe
       expected_hash = { :id => 1, :nested => nested_hash, :list => [nested_hash] }
       assert_equal expected_hash, obj.to_hash
     end
+
+    should "assign question mark accessors for booleans" do
+      obj = Stripe::StripeObject.construct_from({ :id => 1, :bool => true, :not_bool => 'bar' })
+      assert obj.respond_to?(:bool?)
+      assert obj.bool?
+      refute obj.respond_to?(:not_bool?)
+    end
   end
 end