Accept Float::INFINITY in active_record limit clause

This change allows `limit` scope to take Float::INFINITY.

Before this change,

```
> Topic.limit(Float::INFINITY).count
FloatDomainError: Infinity
...
```

just raise an error, though other floating point values are accepted.

```
> Topic.limit(1.0).count
1
```

So this change supports our intuitive programming, I believe. How do you think?