Currently there is no way to pass request scoped information from
the handler to the FileSytem interface. This can be important
to pass credentials or timeout parameters to the FileSystem
operations. Pipe context through the request from
http.Request.Context(). With pre-go1.7 use context.Background().

Custom FileSystem implementations will need to change, but it will
only require a new argument in each of the FileSystem methods.
The change will be trivial to update to.

Fixes golang/go#17658

Change-Id: I7491faf3467ad55db793a68081e074a9b3f9624d
Reviewed-on: https://go-review.googlesource.com/32421Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>