Problem viewing wms layer

Hi, I'm trying to add a wms "operational layer" or "basemap layer" in ArcGIS Viewer for Flex - Application builder v3.6.

But it seems that "viewer" sends requests to wms server with a epsg:102100 that is not recognized from wms server and generate an error.

If I manually substitute epsg:102100 with epsg:3857, for example in a browser dialog, it is all ok. Where can i change that default behaviour in viewer?

Example
Request generated from viewer:

http://www502.regione.toscana.it/wms...REQUEST=GetMap

Correct request:

http://www502.regione.toscana.it/wms...REQUEST=GetMap

thanks in advance
Andrea