Update SvgDocument.cs

1. Added XML Header, this is conform according to https://www.w3.org/TR/SVG/struct.html
2. Added support for removing Byte Order Mark (BOM), this needed in some cases, eg. if the SVG is used as SSI (Server Side Include) in IIS (Internet Information Server).

Source/SvgDocument.cs

@@ -575,12 +575,12 @@ namespace Svg
             }
         }
 
-        public void Write(Stream stream)
+        public void Write(Stream stream, bool useBom = true)
         {
 
-            var xmlWriter = new XmlTextWriter(stream, Encoding.UTF8);
+            var xmlWriter = new XmlTextWriter(stream, useBom ? Encoding.UTF8 : new System.Text.UTF8Encoding(false));
             xmlWriter.Formatting = Formatting.Indented;
-
+            xmlWriter.WriteStartDocument();
             xmlWriter.WriteDocType("svg", "-//W3C//DTD SVG 1.1//EN", "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd", null);
             
             if (!String.IsNullOrEmpty(this.ExternalCSSHref))
@@ -591,11 +591,11 @@ namespace Svg
             xmlWriter.Flush();
         }
 
-        public void Write(string path)
+        public void Write(string path, bool useBom = true)
         {
             using (var fs = new FileStream(path, FileMode.Create, FileAccess.Write))
             {
-                this.Write(fs);
+                this.Write(fs, useBom);
             }
         }
     }