Fixed sheet-scanning effor for #226
Fixed sheet-scanning effor for #226

Proxy was incorrectly using a default value of 0 instead of None when
the sheet index wasn't specified, and that led libhxl to think that
the user wanted a specific sheet.

HXLStandard/hxl-proxy,HXLStandard/hxl-proxy,HXLStandard/hxl-proxy,HXLStandard/hxl-proxy