[release-branch.go1.10] mime/multipart: restore 1.9 handling of missing/empty form-data file name
Revert the code change of CL 70931, but keep the test, appropriately
modified for the code changes. Also add a new test. This restores the
1.9 handling of form-data entries with missing or empty file names.
Changing the handling of this simply confused existing programs for no
useful benefit. Go back to the old behavior.
Updates #19183
Fixes #24041
Change-Id: Ie7a0309a061218ceda3bbc2a7da85e6fb3dd016d
Reviewed-on: https://go-review.googlesource.com/121075
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>