Add fixed date format

To accompany the earlier added fixed locale I've added this PR to add a fixed date format option.

This change also includes change to fixed locale property name. I changed `spring.mvc.locale` to `spring.mvc.fixed.locale` to group the fixed locale and date format.

Note that `noLocaleResolver()` in `WebMvcAutoConfigurationTests` was missing the `@Test` annotation so I added that one as well.


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