Buffer: Use explicit property list

[jng]

Reported here: ​​http://osgeo-org.1560.x6.nabble.com/PostgreSQL-SelectFeatures-td5236983.html

Currently buffer.php does the equivalent of "select * from table", which works fine most of the time, but in the case of the PostgreSQL FDO provider, causes errors if the mapped database table contains columns not supported or recognised by the FDO provider.

The fix is to explicitly specify all the valid properties in the query before executing it.