handle empty products set on cache_key_for_products

Conflicts:
	core/app/helpers/spree/products_helper.rb
rely on maximum return to fall back on static date

Fixes #4565
Add Date string to caching/products_spec
Set RAILS_ENV=test on rake test_app

Got weird issues while building dummy apps in extensions because the
dummy development db, which we don't really care about, was not being
dropped and migrations would always fail