Reactivated 8bit and 1bit mode for QGIS Server

The commit 76d1f95 simplified the
mFormat private attribute to PNG, JPG, PDF, etc and created
mFormatString attribute. The second is used for base64 but not for mode.
This commit corrected it.

src/mapserver/qgshttprequesthandler.cpp

@@ -96,13 +96,13 @@ void QgsHttpRequestHandler::sendGetMapResponse( const QString& service, QImage*
   QgsDebugMsg( "Sending getmap response..." );
   if ( img )
   {
-    bool png8Bit = ( mFormat.compare( "image/png; mode=8bit", Qt::CaseInsensitive ) == 0 );
-    bool png1Bit = ( mFormat.compare( "image/png; mode=1bit", Qt::CaseInsensitive ) == 0 );
+    bool png8Bit = ( mFormatString.compare( "image/png; mode=8bit", Qt::CaseInsensitive ) == 0 );
+    bool png1Bit = ( mFormatString.compare( "image/png; mode=1bit", Qt::CaseInsensitive ) == 0 );
     bool isBase64 = mFormatString.endsWith( ";base64", Qt::CaseInsensitive );
     if ( mFormat != "PNG" && mFormat != "JPG" && !png8Bit && !png1Bit )
     {
       QgsDebugMsg( "service exception - incorrect image format requested..." );
-      sendServiceException( QgsMapServiceException( "InvalidFormat", "Output format '" + mFormat + "' is not supported in the GetMap request" ) );
+      sendServiceException( QgsMapServiceException( "InvalidFormat", "Output format '" + mFormatString + "' is not supported in the GetMap request" ) );
       return;
     }