Merge pull request #1183 from cmarcelo/dbus-improve-unittests

[D-Bus] Improve unittest for PropertyExporter
[D-Bus] Add support for GetAll() in PropertyExporter

Some code was refactored so we could reuse between Get() and
GetAll(). The function AppendPropertiesToWriter() is being exposed
publicly so we can use it to implement GetManagedObjects() in the
future.