Merge pull request #122 from mcolyer/fix-psych-deserialization

Check for `@values` in StripeObject#method_not_found.

This is necessary because we check for `@values` in `respond_to_missing?` now, but this isn't necessarily always set. For example, in deserializing from YAML, Psych creates the object using `allocate` and not `new`, which ends up skipping any initializing of instance variables. See #122 for more details.

lib/stripe/stripe_object.rb

@@ -184,7 +184,7 @@ def method_missing(name, *args)
     end
 
     def respond_to_missing?(symbol, include_private = false)
-      @values.has_key?(symbol) || super
+      @values && @values.has_key?(symbol) || super
     end
   end
 end