[BUGFIX][Server] QgsOWSServer 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/qgsowsserver.cpp

@@ -57,10 +57,9 @@ void QgsOWSServer::restoreLayerFilters( const QHash<QgsMapLayer*, QString>& filt
     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( "Layer does not support Subset String" );
       }
     }
   }