DOC export: restore quotation of SEQ field names

Commit d738b063134e97c780205f8baf138f291016ded4 (fdo#74431 : Captions'
are preserved but remove a space, 2014-02-03) unconditionally removed
quotes around SEQ field names, because the DOCX and RTF import leaves
those quotes around during import time. However, the DOC import strips
them away.

So till all this is unified, at least make sure that on export DOC still
quotes the field names, as it did since the initial import.

Change-Id: I3478f736c2e17824f268f92580767b90d9fdd006