Mixer2ViewResolver should return null if could not resolve view name. #9
Description
http://docs.spring.io/spring/docs/3.1.x/spring-framework-reference/html/mvc.html
"The contract of a view resolver specifies that a view resolver can return null to indicate the view could not be found."
Now, Mixer2ViewResolver throws FileNotFoundException when the view could not be found.
Mixer2ViewResolverがviewを処理できない場合に後続のVeiwResolver（例えばjsp用のinternalResourceViewResolver)に引き継げるようにする。