buildlet: add WorkDir method on Client

Also fix the RemoveAll method by setting the request Content-Type.

Change-Id: I87ec29c5c0da06eba5eaebcd00bdbef18e6ae8ad
Reviewed-on: https://go-review.googlesource.com/3880
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed