hstore-new - hstore-new: Deal with NULL issues in populate_record.

Log Message:
-----------
Deal with NULL issues in populate_record. We cope with the various issues
as follows:

 - if we're passed in a null datum, we ensure that every field in the
   returned heaptuple has been through its input function, even the null
   values for fields that weren't set in the hstore

 - if we're passed in a non-null datum heaptupleheader, we assume it's the
   caller's job to have done any validation, domain checks, whatever, and
   so we only call the input function on columns that we're overriding
   with data from the hstore (even if the overriding value is a null)

Add appropriate regression tests.

Modified Files:
--------------
    hstore-new:
        hstore_io.c (r1.5 -> r1.6)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/hstore-new/hstore-new/hstore_io.c?r1=1.5&r2=1.6)
    hstore-new/expected:
        hstore.out (r1.5 -> r1.6)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/hstore-new/hstore-new/expected/hstore.out?r1=1.5&r2=1.6)
    hstore-new/sql:
        hstore.sql (r1.5 -> r1.6)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/hstore-new/hstore-new/sql/hstore.sql?r1=1.5&r2=1.6)