Should Series.__contains__ look at indexes or values?
When writing this SO answer, I came upon this behavior, which I did not expect.

```
In [1]: 1 in Series([1])
Out[1]: False

In [2]: 1 in Series([1, 2])
Out[2]: True
```

Apparently, `__contains__` looks at the index.

```
In [3]: 1 in Series([1], index=[1])
Out[3]: True
```

I can see how that makes sense. But I think it's fair to say that it is potentially confusing. Do we rely on this behavior of `__contains__` for data alignment? Might it be reasonable to change it? I'm not sure that's a good idea, but I want to raise the issue.
