[MERGE] fix xml5 serializer with empty property values

[John Arbash Meinel]

The current serializer expects all revision properties to have string
keys and values when writing out.

However, because of how they are stored, when reading in, an empty
string is translated into None. (they are the 'text' property, and there
is no text for the empty string).

The attached patch adds a test for this, and a fix.

It also makes sure that revision properties can be round tripped. Which
is something that Jamie might need to update with his Graft plugin.
I don't know if he is copying revision properties or not, but since he
wrote the test, and he wasn't testing for it, I'm guessing he isn't.

This probably fixes bug:
https://launchpad.net/products/bzr/+bug/47782

But I would need to actually use that branch to know for sure. (But
judging on where it is failing, that should be the case)

John
=:->
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