Now, instead of saying
X-Versions-Location: <container>
X-Versions-Mode: history
clients should just say
X-History-Location: <container>
Since we've never had a release featuring a user-settable
X-Versions-Mode header, support may be dropped and that is now ignored.
Change-Id: Icfd0f481d4e40dd5375c737190aea7ee8dbc3bf9