futures_of returns empty list for dict values input

Definitely debatable whether this is a bug or not but to me it seems undesirable:
```
[ins] In [2]: c = distributed.Client()

[nav] In [3]: f = c.submit(sum, [1, 2])

[ins] In [4]: distributed.futures_of([f])
Out[4]: []

[ins] In [5]: distributed.futures_of({"f": f})
Out[5]: []

[ins] In [6]: distributed.futures_of({"f": f}.values())
Out[6]: []
```
I was going to just make a PR but I'm actually not sure what the best way to test that something is a dict view......? If there's not a super clean way of doing it then maybe it's not worth the complexity. But in that case it would be nice if we could raise an exception here instead of incorrectly returning `[]`.