[ttx] Always add raw=True attribute when dumping DefaultTable to XML

Needed to avoid surprises when we add new table implementations.
Recently we added VDMX table, and that broke build of projects using
VDMX with DefaultTable, eg:

https://github.com/behdad/fonttools/issues/151

Lib/fontTools/ttLib/__init__.py

@@ -307,10 +307,13 @@ class TTFont(object):
 		if tag not in self:
 			return
 		xmlTag = tagToXML(tag)
+		attrs = dict()
 		if hasattr(table, "ERROR"):
-			writer.begintag(xmlTag, ERROR="decompilation error")
-		else:
-			writer.begintag(xmlTag)
+			attrs['ERROR'] = "decompilation error"
+		from .tables.DefaultTable import DefaultTable
+		if table.__class__ == DefaultTable:
+			attrs['raw'] = True
+		writer.begintag(xmlTag, **attrs)
 		writer.newline()
 		if tag in ("glyf", "CFF "):
 			table.toXML(writer, self, progress)