280: Copy PropertyGroup API broken for uneven collections

[tngreene]

Discovered while filing #510.

While e67cf51 allows new Blender 2.80 projects to use Collections, the updater's property group copy API is broken.

for source_member, dest_member in zip(source_members, dest_members):

if source_members (a collection of say 4 LR directives) and dest_members (0, of course) then nothing is copied. This API was developed for two collections that were always going to be the same length. See #493 also for when we'll fix this.

The fix is simple: while len(source_members) > dest_members.add(). If we have to remove any by that point, something is wrong. dest_members should be empty or perfect from other update utils.

[tngreene]

I think this is breaking various.test's custom header properties. I've corrected the test to make up for the updater's fault, but, I think this is an example of not copying a list.