dictproxy iteration does not yield descriptors

Descriptors are omitted from dictproxy iteration.  See ipy console session:

```
IronPython 2.7.3 (2.7.0.40) on Mono 4.0.30319.17020 (64-bit)
Type "help", "copyright", "credits" or "license" for more information.
>>> class Descriptor(object):
...  def __get__(self, instance, owner): return instance.x    
... 
>>> class X(object): a = Descriptor()
... 
>>> X.__dict__
<dictproxy object at 0x000000000000003E>
>>> X.__dict__['a']
<Descriptor object at 0x000000000000003D>
>>> X.__dict__.keys()
['__doc__', '__weakref__', '__dict__', '__module__']
```

The expected behaviour is that descriptors would appear in dictproxy iteration.