replication: keep header when request is modified by before_replace

When space.before_replace trigger modifies the result of a remote
operation, we clear the request header so that it gets rebuilt on
commit. This is incorrect, because as a result we don't bump the
master's component of the replica's vclock, which leads to the request
being applied again when the replica reconnects. The issue manifests
itself in sporadic replication/before_replace test failures.

Fix it by updating the request header rather than clearing it so that
replica id and lsn get preserved.

Closes #3722