Date SDT's are normally imported as form controls, while most other SDT
types are just custom properties on regular text portions or paragraphs.

However, given that form controls are not supported in headers/footers,
in that case even date SDT's are just custom properties. So support such
properties on paragraphs in the exporter to properly roundtrip date
SDT's in headers/footers.

Change-Id: I19eb73a3673e387a7b8780756ce7426a1851e796