remove disable_with if not needed

gluon/html.py

@@ -1482,8 +1482,11 @@ class A(DIV):
     def xml(self):
         if not self.components and self['_href']:
             self.append(self['_href'])
-        if not self['_disable_with']:
-            self['_data-w2p_disable_with'] = 'default'
+        disable_needed = ['callback', 'cid', 'delete', 'component', 'target']
+        disable_needed = any((self[attr] for attr in disable_needed))
+        if disable_needed:
+            self['_data-w2p_disable_with'] = self['_disable_with'] or 'default'
+            self['_disable_with'] = None
         if self['callback'] and not self['_id']:
             self['_id'] = web2py_uuid()
         if self['delete']:
@@ -2518,7 +2521,7 @@ def test():
 
     >>> from validators import *
     >>> print DIV(A('click me', _href=URL(a='a', c='b', f='c')), BR(), HR(), DIV(SPAN("World"), _class='unknown')).xml()
-    <div><a data-w2p_disable_with="default" href="/a/b/c">click me</a><br /><hr /><div class=\"unknown\"><span>World</span></div></div>
+    <div><a href="/a/b/c">click me</a><br /><hr /><div class=\"unknown\"><span>World</span></div></div>
     >>> print DIV(UL("doc","cat","mouse")).xml()
     <div><ul><li>doc</li><li>cat</li><li>mouse</li></ul></div>
     >>> print DIV(UL("doc", LI("cat", _class='feline'), 18)).xml()