Join overlapping areas not correctly forming multipolygons

[pnorman]

When joining two identically tagged closed ways where the resulting shape has an internal void, a multipolygon is created. Unfortunately, the tagging of the ways is not carried over to the multipolygon but remains on the outer way.

To reproduce:

1. Draw two areas that overlap and when joined will have an internal void.
2. Tag both ways identically (e.g. building=yes)
3. Select both ways and do join overlapping areas

Expected result: Two untagged ways that are members of a multipolygon with the tagging (e.g. building=yes)
Actual result: Two ways as members of a multipolygon with no tagging, and a tag on the outer way.

[Don-vip]

Example data