Prior to this commit the MockHttpServletRequest constructor chain set
the preferred local to Locale.ENGLISH. Furthermore, it was possible to
add additional preferred locales "in front" of ENGLISH; however, it was
not possible to delete ENGLISH from the list of preferred locales.
This commit documents the fact that ENGLISH is the default preferred
locale and makes it possible to set the list of preferred locales via a
new setPreferredLocales(List<Locale> locales) method.
Issue: SPR-9724
591aa01741