Fix tag names generated in API XML output.

This fixes ResultBuilder misbehaving if a serializer already set
the magic _element field for specifying the tag name to use in XML
output.

Deploy: this is technically a breaking chance, since it changes
the structure of the API's XML output; However, the previous output
was obviously broken and should not be considered canonical.
So this is really an un-breaking change.

Bug: 70531
Change-Id: I98922c0f0566f57e4a8dc88f69c8ef4023d2e3ea
Merge "Fix tag names generated in API XML output." into mw1.24-wmf19