[BUGFIX][Server] Filter restorer modifying layers and not well restoring them

The Subsetstring is updated through QgsVectorLayer::setSubsetString and restored by QgsVectorDataProvider::setSubsetString and it's not idempotent.
The solution is to use QgsVectorLayer::setSubsetString in the 2 cases.

Author: rldhont

src/server/qgsfilterrestorer.cpp

@@ -61,10 +61,9 @@ void QgsOWSServerFilterRestorer::restoreLayerFilters( const QHash<QgsMapLayer *,
     QgsVectorLayer *filteredLayer = qobject_cast<QgsVectorLayer *>( filterIt.key() );
     if ( filteredLayer )
     {
-      QgsVectorDataProvider *dp = filteredLayer->dataProvider();
-      if ( dp )
+      if ( !filteredLayer->setSubsetString( filterIt.value() ) )
       {
-        dp->setSubsetString( filterIt.value() );
+        QgsMessageLog::logMessage( QStringLiteral( "Layer does not support Subset String" ) );
       }
     }
   }