Fixed metadata serialization of Numbers and DNs

There were following problems:
1. DNs and Decimals weren't properly serialized. Serialization output was object with empty __base64__ attribute. It was fixed by converting them to string.
2. numberical values equal to 0 were excluded from metadata. It broke many of minvalue checks in Web UI. Now excluding only None and False values as initally intended.

https://fedorahosted.org/freeipa/ticket/3052

ipalib/util.py

@@ -26,6 +26,7 @@ import imp
 import time
 import socket
 import re
+import decimal
 from types import NoneType
 from weakref import WeakKeyDictionary
 from dns import resolver, rdatatype
@@ -46,6 +47,8 @@ def json_serialize(obj):
         return obj
     if isinstance(obj, str):
         return obj.decode('utf-8')
+    if isinstance(obj, (decimal.Decimal, DN)):
+        return str(obj)
     if not callable(getattr(obj, '__json__', None)):
         # raise TypeError('%r is not JSON serializable')
         return ''