Examining a defaultdict adds a key of 0

Run pudb on this code:

``` python
import collections
d = collections.defaultdict(set)
print d
print len(d)
```

It will print:

```
defaultdict(, {})
0
```

Now run the code in pudb again, but after d is created, open it in the Variables pane. It will have a key of 0 with an empty set as a value, and when the program is finished running, it will have printed:

```
defaultdict(, {0: set([])})
1
```

Start by reproducing the supplied Python snippet in pudb and opening the defaultdict in the Variables pane. Trace how the Variables pane examines defaultdict values, then verify that inspecting d does not add key 0 and that its length and printed contents remain unchanged.

Written by the indexing model from the issue text.