[Bug-wget] Timestamping and --continue

[Jack Bates]

When combined with the --continue option, should --timestamping send an 
If-Unmodified-Since header?

Currently, if I partially download a file and then run `wget --debug 
--continue --timestamping URL` I get:

> ---request begin---
> GET [...] HTTP/1.1
> If-Modified-Since: Tue, 13 Nov 2018 14:13:29 GMT
> Range: bytes=1084899556-
> [...]
>
> ---response begin---
> HTTP/1.1 304 Not Modified
> [...]
>
> ---response end---
> 304 Not Modified
> File ‘[...]’ not modified on server. Omitting download.

and I'm left with the still-incomplete file 😞

If Wget instead sent If-Unmodified-Since, I'd expect it to resume 
downloading:

> ---response begin---
> HTTP/1.1 206 Partial Content
> [...]
>
> ---response end---
> 206 Partial Content
> Length: [...], [...] remaining
> Saving to: ‘[...]’

If the file was already complete, I'd expect:

> ---response begin---
> HTTP/1.1 416 Requested Range Not Satisfiable
> [...]
>
> ---response end---
> 416 Requested Range Not Satisfiable
> The file is already fully retrieved; nothing to do.

And if the file was out of date (complete or otherwise), I'd expect:

> ---response begin---
> HTTP/1.1 412 Precondition Failed
> [...]
>
> ---response end---
> 412 Precondition Failed