JIRA: NCF-308

Jennifer Adams has requested a reversion in behavior so that
all dap requests include a constraint.

Problem is caused by change in prefetch where if all variables
are requested, then no constraint is generated.
Fix is to always generate a constraint in prefetch.
Merge branch 'master' of http://github.com/Unidata/netcdf-c
Merge branch 'master' into ncf304