fixes converting uidl to xml string. Visible in some error messages.

svn changeset:8693/svn branch:6.1

src/com/vaadin/terminal/gwt/client/UIDL.java

@@ -257,7 +257,7 @@ public final class UIDL extends JavaScriptObject {
                 buf.push("tag");
                 buf.push(">");
             }
-            return buf.join();
+            return buf.join("");
         }-*/;
     }