Client.submit does not use the custom serialization

The example below explains it well I think, I can scatter a hdf5 file instance, but not use it directly as argument, which is a bit unexpected.
```python
import h5py
f = h5py.File('/Users/maartenbreddels/datasets/nytaxi/nyc_taxi2015.hdf5', 'r')
def echo(f):
return f.filename
f_future = client.scatter(f)
#client.submit(echo, f).result() # this fails, it tries to use pickle
client.submit(echo, f_future).result() # this works fine
```
Is there a reason for this, or is this a bug, or missing feature?