In https://golang.org/3210, Transport errors occurring before
receiving response headers were wrapped in another error type to
indicate to the retry logic elsewhere that the request might be
re-tryable. But a check for err == io.EOF was missed, which then became
false once io.EOF was wrapped in the beforeRespHeaderError type.

The beforeRespHeaderError was too fragile. Remove it. I tried to fix
it in an earlier version of this CL and just broke different things
instead.

Also remove the "markBroken" method. It's redundant and confusing.

Also, rename the checkTransportResend method to shouldRetryRequest and
make it return a bool instead of an error. This also helps readability.

Now the code recognizes the two main reasons we'd want to retry a
request: because we never wrote the request in the first place (so:
count the number of bytes we've written), or because the server hung
up on us before we received response headers for an idempotent request.

As an added bonus, this could make POST requests safely re-tryable
since we know we haven't written anything yet. But it's too late in Go
1.7 to enable that, so we'll do that later (filed #15723).

This also adds a new internal (package http) test, since testing this
blackbox at higher levels in transport_test wasn't possible.

Fixes #15446

Change-Id: I2c1dc03b1f1ebdf3f04eba81792bd5c4fb6b6b66
Reviewed-on: https://go-review.googlesource.com/23160Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>