There are cases where HTTP message specifies the Trailer header
but not the Transfer-Encoding = chunked. The existing
implementation would return an error in those cases, without
returning also the message itself.
Instead, it would be preferable to let the library user decide when
the message is valid or not.
This change makes the fixTrailer() function not to return an error
and to keep the Trailer value in the Response.Header map but not
populate Response.Trailer.

Fixes #27197

Change-Id: Ic1e96791fde97f31dc5ecb8de05c8e4f49465c2c
Reviewed-on: https://go-review.googlesource.com/c/145398
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>