When returning type "record", replace use of pgresultGetTupleDesc

When returning type "record", replace use of pgresultGetTupleDesc
with ReturnSetInfo->expectedDesc. This allows custom datatypes
(e.g. from tsearch2) to be returned at runtime. Previous behavior
depended on the type oid to match between the remote and local
database, which obviously doesn't work well for custom types.
Per report from Mark Gibson.