__in__ appears always returns False

It should error out or return the correct result.

```
(Pdb) self.store["blah"] = "1"
(Pdb) "blah" in self.store
False
(Pdb) self.store["blah"]
b'1'
(Pdb) self.store
<semidbm.db._SemiDBM object at 0x11f6ec050>
(Pdb) semidbm.__version__
'0.5.1'
```