1634 dont return none 5

OK, here's a revised patch:

(0) The test module name is all lower case.
(1) The tests are refactored to increase DRYness with use of mixin
(2) self.failFOO('size', metdata.keys()) --> self.failFOO('size', metadata)
(3) The case of size 0 is correctly differentiated from size "None".

Thanks for the review Zooko!
P.S.-- I need to add a new email address to the tahoe-dev mailing list ASAP, so any list admins who read this should act on my request!