[BUGFIX] Set default units to mm in exportSld

The default units in QgsMapRenderer is Millimeters. The default units in SLD
 is Pixel but exportSld does not convert all millimeters in pixels.

We notes this bug by comparing default QGS Server rendering and rendering with
 the SLD generated by QGIS.

To resolve it, we just have to add units to the document element.

Author: rldhont

src/core/qgsmaplayer.cpp

@@ -1370,6 +1370,7 @@ void QgsMapLayer::exportSldStyle( QDomDocument &doc, QString &errorMsg )
   // Create the root element
   QDomElement root = myDocument.createElementNS( "http://www.opengis.net/sld", "StyledLayerDescriptor" );
   root.setAttribute( "version", "1.1.0" );
+  root.setAttribute( "units", "mm" ); // default qgsmaprenderer is Millimeters
   root.setAttribute( "xsi:schemaLocation", "http://www.opengis.net/sld http://schemas.opengis.net/sld/1.1.0/StyledLayerDescriptor.xsd" );
   root.setAttribute( "xmlns:ogc", "http://www.opengis.net/ogc" );
   root.setAttribute( "xmlns:se", "http://www.opengis.net/se" );