When using fast-post and POST (i.e. metadata update) is requested to
a SLO manifest files, current Swift drops the 'X-Static-Large-Object'
header from the existing metadata. It results in breaking the SLO
state because the manifest missing the 'X-Static-Large-Object' metadata
will be maintained as a normal files.
This patch fixes object-server to keep the existing
'X-Static-Large-Object' flag and then keep the SLO state.
Change-Id: Ib1eb569071372c322dd105c52baeeb094003291e
Closes-bug: #1453807