QDateTime - Fix RFC 2822 Date Formatting

The RFC 2822 date format should always use en_US locale for month and
day names instead of whatever the system locale is.  Also remove some
duplicate code.

Change-Id: Ia2f7ee405b4e0e2f04980301783b9488628da73f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>