Make DLO manifest copying work with ?multipart-manifest=get

If a client issues a COPY request for a DLO with the query param
multipart-manifest=get, then the expectation is a new
manifest. However, this wasn't the case; X-Object-Manifest wasn't
being set on the new object, so the result was a normal object with
the same contents as the old manifest (typically 0 bytes).

There was already a mechanism by which middlewares could modify COPY
requests; this commit extends that so they can set headers on the new
object.

Note that this has nothing to do with a "normal" DLO copy, i.e. one
without multipart-manifest=get. That one makes a new object that's the
concatenation of the segments, and it was working just fine.

Change-Id: I1073af9fee6e34ebdfad7b1a89aeb05e4523a151
Merge "Make DLO manifest copying work with ?multipart-manifest=get"