Add a helper method for temporarily setting I18n.default_locale

Although the I18n library has an I18n.with_locale helper, there's
no equivalent for I18n.default_locale.  This method will make a
couple of our tests clearer and less brittle.
Refactor some tests to use with_default_locale
Fix construction of example public bodies for upstream globalize

We're trying to switch to an upstream version of globalize, which now
has Henare's change to allowed blank values without them being
converted to nil - however, there is another intermediate change that
was introduced which means that the name is only written back to the
original table if the locale matches the default locale.  This is
described in more depth here:

   https://github.com/globalize/globalize/pull/183

This change broke the make_single_language_example helper, but this
commit fixes that by also setting the default locale before creating
the example public bodies.
Merge branch 'fixes-for-upstream-globalize' into rails-3-develop