Add comment that the check is based on the stdlib HTTP2 client. Refactor
the checks into a function. Return an error if the value in the
Content-Length header cannot be parsed.
The first test function ensures that the workaround works as expected.
And the second test function is intended to fail as soon as the issue
has been fixed in golang to allow us to eventually remove the
workaround.
The golang http client does not return an error when a HTTP2 reply
includes a non-zero content length but does not return any data at all.
This scenario can occur e.g. when using rclone when a file stored in a
backend seems to be accessible but then fails to download.
Before this change restic would attempt to JSON decode the error
message resulting in confusing `Decode: invalid character 'B' looking
for beginning of value` messages. Afterwards it will return `List
failed, server response: 400 Bad Request (400)`
As mentioned in issue [#1560](https://github.com/restic/restic/pull/1560#issuecomment-364689346)
this changes the signature for `backend.Save()`. It now takes a
parameter of interface type `RewindReader`, so that the backend
implementations or our `RetryBackend` middleware can reset the reader to
the beginning and then retry an upload operation.
The `RewindReader` interface also provides a `Length()` method, which is
used in the backend to get the size of the data to be saved. This
removes several ugly hacks we had to do to pull the size back out of the
`io.Reader` passed to `Save()` before. In the `s3` and `rest` backend
this is actively used.
Before, creating a new repo via REST would use the defaut HTTP client,
which is not a problem unless the server uses HTTPS and a TLS
certificate which isn't signed by a CA in the system's CA store. In this
case, all commands work except the 'init' command, which fails with a
message like "invalid certificate".
