The acl parameter of open() is ignored when uploading an object smaller than blocksize

What happened:

with s3.open(s3_uri, 'wb', acl='bucket-owner-full-control') as f:
    f.write(b'x' * (5 * 2 ** 20 - 1))

does not obey the acl parameter (the resulting owner is incorrect), whereas

with s3.open(s3_uri, 'wb', acl='bucket-owner-full-control') as f:
    f.write(b'x' * (5 * 2 ** 20))

does.

What you expected to happen:

For the acl parameter to work regardless of object size.

Minimal Complete Verifiable Example:

with s3.open(s3_uri, 'wb', acl='bucket-owner-full-control') as f:
    f.write(b'x' * (5 * 2 ** 20 - 1))

Anything else we need to know?:

I tested this with s3fs 0.4.2 because I can't currently install 0.5.x (dependency issues), but I suspect it is broken even on current master, because it's probably this if-statement causing the use of the single-part code path that does not use self.acl the way the multi-part code path does.

Environment:

• Dask version: 2.11.0
• Python version: 3.6.10
• Operating System: Linux (on SageMaker)
• Install method (conda, pip, source): It's SageMaker (conda) with a bunch of packages installed on top with pip, I'm not sure what exactly gets in as dependencies (but one is explicit s3fs<0.5). If this turns out to matter, I'll look into getting a cleaner install, but it probably doesn't.

Start in s3fs/core.py at the if-statement around line 1415 and compare the single-part path around line 1535 with the multipart path around line 1422. Reproduce the reported behavior with objects just below and at 5 MiB, then verify that the acl parameter is honored for both paths.