Revert the code changes of CL 96975 and CL 70931, but keep the tests,
appropriately modified for the code changes. 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: I4ebc32433911e6360b9fd79d8f63a6d884822e0e
Reviewed-on: https://go-review.googlesource.com/121055
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>