Fallback to "en_US" if browser does not send preferred languages

Fallback to "en_US" if browser does not send preferred languages

E.g. in our test suite we don't send an "accepted languages" header, which
caused the language matching to fail. So we need to explicitely fallback to
en_US, in case request.accepted_languages is None. This fixes the tests and
all cases where user browsers don't send preferred languages.

This also fixes issue #562, the AVAILABLE_LOCALES are already case-normalized
and we don't need to fudge the preferred language through the lower_upper_locale
thing for each and every request.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>