Fix filter with new opaque key, allowing user to set COURSE_LISTING as o...

...ld way path


Alllow settings as: 
COURSE_LISTINGS = {
    'default': ['BerkeleyX/CS169.1x/2012_Fall',
                'BerkeleyX/CS188.1x/2012_Fall',
                'HarvardX/CS50x/2012',
                'HarvardX/PH207x/2012_Fall',
                'MITx/3.091x/2012_Fall',
                'MITx/6.002x/2012_Fall',
                'MITx/6.00x/2012_Fall'],
    'berkeley': ['BerkeleyX/CS169/fa12',
                 'BerkeleyX/CS188/fa12'],
    'harvard': ['HarvardX/CS50x/2012H'],
    'mit': ['MITx/3.091/MIT_2012_Fall'],
    'sjsu': ['MITx/6.002x-EE98/2012_Fall_SJSU'],
}

and filter course correctly with new opaque key