allow the cdmi value request arg also without a range

When 'value' is passed and it's value (the key being 'value' ...)
is initially None, i.e. it's passed as:
    host.com/path?value
and not
    host.com/path?value:2-10
then return
    ('value', [])
from the cdmi_filter function so that the get function
does not try to do a range request