default querying filter

It'd be nice for filtering to default to the `__eq` operator

_s.t._ this:

```
>>> Token.items.query(userid=1)
```

works just like this:

```
>>> Token.items.query(userid__eq=1)
[…]
```

rather than:

```
>>> Token.items.query(userid=1)
…
UnknownFilterTypeError: Operator 'userid' from 'userid' is not recognized.
```
