Tweak handling of acquisitionDate

In order to make the choice and use of import v.
acquisition date more natural:

 * import (i.e. creation date) is used by default
 * for non-Image types, import date is used if the
   user requests "acquisitionDate" (server-side)
Merge pull request #2823 from joshmoore/search-acq-types

Tweak handling of acquisitionDate