Add support for date data type

[imshubhangi]

Following error is seen , when there is date type of column in greenplum database and JSON document contains date represented in String type.

ext_web_formatter_json=# select * from accounts; ERROR: Unsupported data type '1082' for column 'birthdate' (seg0 slice1 127.0.0.1:40000 pid=19733) DETAIL: External table accounts

Look like , in json_formatter.c date data type is not handled , it goes to default case in switch(type) and errors out.

Requesting to add support for date data type.