http: allow put_file()ing file-like objects

[isidentical]

Since writing to HTTPFile is not supported, currently you can not upload arbitrary streams (unlike other filesystems where you could open a new file in writing mode through fs.open(path, 'wb') and copy the original stream through shutil.copy_file_obj() to the new file). Not sure if this would be worth documenting at the moment, but would be extremely useful to compensate for the missing w method at the moment while we are thinking about other solutions.

[martindurant]

This seems like it might be a useful thing to do for all filesystems.

Possibly related: #723

[isidentical]

I agree. It might require a bigger design discussion, but something I wondered when I started to the migration to the fsspec was why this was not supported natively (since that would potentially render the requirement to the callbacks redundant, since we could simply wrap read() or write() methods of the original stream with the callback updates).

[martindurant]

Should be callback.set_size(getattr(f, "size", None)) ? Or if hasattr(f, "size"):

[martindurant]

I think this one was right!
It says, "get file from remote to local", and should be the revers of put_file. That's how we have it in spec.

[isidentical]

Ah, missed that! Thanks for the correction.