Mixer2ViewResolver should return null if could not resolve view name.

[nabedge]

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.

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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。

[nabedge]

fix on 1.2.21

[nabedge]

1.2.21 still have bug.
fix on 1.2.22