If we were to include this in a normal PUT, it would 400, but only if
slo is actually in the pipeline. If it's *not*, we'll create a normal
Swift object and the header sticks.
- This is really confusing for users; see the related bug.
- If slo is later enabled in the cluster, Swift starts responding 500
with a KeyError because the client and on-disk formats don't match!
Change-Id: I1d80c76af02f2ca847123349224ddc36d2a6996b
Related-Change: I986c1656658f874172860469624118cc63bff9bc
Related-Bug: #1680083