[release-branch.go1.6] net/http: multipart ReadForm close file after copy

Always close the file regardless of whether the copy succeeds or fails.
Pass along the close error if the copy succeeds

Updates #16296

Change-Id: Ib394655b91d25750f029f17b3846d985f673fb50
Reviewed-on: https://go-review.googlesource.com/30410


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/33640


Reviewed-by: Chris Broadfoot <cbro@golang.org>