Set the default resolution of wms provider to 256*256

[leodai]

Some WMS servers don't support 1024 pixel size. So if you execute a select command which doesn't have Identifier like "Resample" or "Clip", it will use the default resolution to query the data. And then the server may return a exception to the client. But mostly the select command without Identifier is just used for getting some query information like extents and bands. In another word, high resolution is no useness in this step.