Add ToXDocument method to TwiML (#649)

src/Twilio/TwiML/TwiML.cs

@@ -154,17 +154,25 @@ protected virtual XNode ToXml()
 
             return elem;
         }
+
+        /// <summary>
+        /// Generate XDocument from TwiML object
+        /// </summary>
+        public XDocument ToXDocument()
+        {
+            var declaration = new XDeclaration("1.0", "utf-8", null);
+            var elem = this.ToXml();
+            var document = new XDocument(declaration, elem);
+            return document;
+        }
 
         /// <summary>
         /// Generate XML string from TwiML object
         /// </summary>
         /// <param name="formattingOptions"> Change generated string format. </param>
         public string ToString(SaveOptions formattingOptions = SaveOptions.None)
         {
-            var declaration = new XDeclaration("1.0", "utf-8", null);
-            var elem = this.ToXml();
-
-            var document = new XDocument(declaration, elem);
+            var document = this.ToXDocument();
             var writer = new Utf8StringWriter();
             document.Save(writer, formattingOptions);
             return writer.GetStringBuilder().ToString();