Preserve <span> information in MultiText strings

This fixes http://jira.palaso.org/issues/browse/WS-34799, at least
to a first approximation.  No attempt is made to keep the spans in
sync with any editing of the field.  The user is not shown anything
to indicate that spans exist underneath to affect the formatting.
It would be a little bit nicer if the three lines responsible for writing the span were all wrapped into a single method here and below. I can't imagine any place where you would want to write the start element and get the text where you wouldn't also want to write the text and close the element.