Fix Jira TDS-320

Two problems:
1. projecting out a single field of a grid
   was not properly wrapping it as a Structure.

2. projecting out the array variable plus
   only the last map variable produced a
   malformed grid rather than wrapping
   those variables in a structure.
Merge branch 'target-4.3.16' of https://github.com/Unidata/thredds into target-4.3.16