Fix the failing spec from webmock/1.20.2

WebMock fixed argument handling of 'to_return'. Update td-client-ruby's
spec to follow the change. The detail is here; bblimke/webmock#427

spec/td/client/result_api_spec.rb

@@ -12,7 +12,7 @@
   describe 'create_result' do
     it 'should create a new result' do
       params = {'url' => result_url}
-      stub_api_request(:post, "/v3/result/create/#{e(result_name)}").with(:body => params).to_return(:body => {'result' => result_name})
+      stub_api_request(:post, "/v3/result/create/#{e(result_name)}").with(:body => params).to_return(:body => {'result' => result_name}.to_json)
 
       api.create_result(result_name, result_url).should be true
     end