refactor get_filenode_metadata(...) into common.py
exercise all branches of new helper "get_filenode_metadata"

Note fix following issues from origial commit:

refactor unittests, fix style, add test

(0) use CommonFixture as mixin to increase DRYness
(1) self.failUnlessIn('size', metadata.keys()) --> self.failUnlessIn('size', metdata)
(2) test_size_is_not_None --> test_size_is_0 AND test_size_is_1000
Do not report 'size' metadata if get_size returns None
Merge pull request #120 from zancas/1634-dont-return-none_5

1634 dont return none 5