tolerate empty string result from Fiken API

lib/fiken/object.rb

@@ -12,7 +12,8 @@ module Fiken
   class Object
     def initialize(attributes = {})
       @attributes = {}
-      (attributes || {}).each { |key, value| @attributes[key.to_sym] = wrap(value) }
+      attributes = {} unless attributes.is_a?(Hash)
+      attributes.each { |key, value| @attributes[key.to_sym] = wrap(value) }
     end
 
     def [](key)

spec/fiken/object_spec.rb

@@ -46,4 +46,8 @@ RSpec.describe Fiken::Object do
     expect(object.key?(:invoiceNumber)).to be(true)
     expect(object.key?(:missing)).to be(false)
   end
+
+  it "treats a non-Hash body (e.g. empty string from a 204 response) as empty" do
+    expect(described_class.new("").keys).to eq([])
+  end
 end