[composer] Make $rownum in attribute table column expressions evaluat…

…e to current row number (works correctly only if no sort columns are specified - in future release sorting for attribute tables should be reworked so that it is done prior to fetching features)

src/core/composer/qgscomposerattributetable.cpp

@@ -442,6 +442,7 @@ bool QgsComposerAttributeTable::getFeatureAttributes( QList<QgsAttributeMap> &at
       {
         // Lets assume it's an expression
         QgsExpression* expression = new QgsExpression(( *columnIt )->attribute() );
+        expression->setCurrentRowNumber( counter + 1 );
         expression->prepare( mVectorLayer->pendingFields() );
         QVariant value = expression->evaluate( f ) ;
         attributeMaps.last().insert( i, value.toString() );