Not specifying status results in error
Creating a prefix with pynipap results in an error being thrown from the back end if 'status' is not explicitly specified:

```
In [4]: p = pynipap.Prefix()
In [5]: p.prefix = '1.3.3.0/24'
In [6]: p.type = 'reservation'
In [7]: p.description = 'Test'
In [8]: p.save()
---------------------------------------------------------------------------
NipapError                                Traceback (most recent call last)
<ipython-input-8-395e325a0c83> in <module>()
----> 1 p.save()

/home/lukagarb/dev/venv/lib/python2.7/site-packages/pynipap.pyc in save(self, args)
   1029                     })
   1030             except xmlrpclib.Fault as xml_fault:
-> 1031                 raise _fault_to_exception(xml_fault)
   1032
   1033         # Old object, edit

NipapError: null value in column "status" violates not-null constraint
DETAIL:  Failing row contains (5625, 0, 1.3.3.0/24, 1.3.3.0/24, Test, null, null, null, reservation, 0, null, null, null, null, asdf, null, null, 0, null, {}, {}, 2014-11-07 11:04:21.945863+01, 2014-11-07 11:04:21.945863+01, 256, 0, 256, , null, infinity).
In [9]:
```

The expected behavior would IMHO be to have the prefix added with a sane default, such as 'assigned'. I'd say that the backend handles this correctly by throwing an error; pynipap should not pass the 'status' attribute to the add_prefix function when it's not been set.

diff --git a/pynipap/pynipap.py b/pynipap/pynipap.py
index 01ab3c94..e52979f4 100644
--- a/pynipap/pynipap.py
+++ b/pynipap/pynipap.py
@@ -981,10 +981,13 @@ class Prefix(Pynipap):
             'alarm_priority': self.alarm_priority,
             'monitor': self.monitor,
             'vlan': self.vlan,
-            'status': self.status,
             'avps': self.avps,
             'expires': self.expires
         }
+
+        if self.status is not None:
+            data['status'] = self.status
+
         for tag_name in self.tags:
             data['tags'].append(tag_name)
 
['pynipap/pynipap.py']