#26 Unknown fields do not survive an import-export cylce

[Lutz Ißler]

If there are unknown fields in a file that is imported,
these fields are not imported. When the bibliography is
then exported, the information is missing in the output.

I.e. information from unknown fields (eg. when the
plugin that sets up those fields is not installed,
temporarily deactivated or could not be loaded) does
not survive an import-export cycle.