In HGAssetMapper.RewriteSOP(), don't explicitly end the document.

The document here has no <?xml declaration.  Though ideally it may be present, in practice it has not been added as the data here is embedded within an asset
On Mono 3.2.8 and probably other versions, calling WriteEndDocument() without a declaration does nothing.
On Windows .NET, the same operation throws a "Document does not have a root element" exception which may be contributing to Mantis 7397 and possibly 7439

OpenSim/Region/CoreModules/Framework/InventoryAccess/HGAssetMapper.cs

@@ -338,8 +338,6 @@ namespace OpenSim.Region.CoreModules.Framework.InventoryAccess
             {
                 TransformXml(reader, writer);
 
-                writer.WriteEndDocument();
-
 //                Console.WriteLine("Output: [{0}]", sw.ToString());
 
                 return sw.ToString();