[BUG] Custom headers in `notmuch-message-headers` are broken

[Jan Malakhovski]

# What I did

I added "X-Github-Sender" to `notmuch-message-headers`.
Looked at a message sent via github with `notmuch-show`.

# What I expected

To see "X-Github-Sender" header displayed in `notmuch-show`.

# What I got

No such header was displayed.

# Why 

`(notmuch-show "query")` runs

```
notmuch show --format=sexp --format-version=4 query
```

internally. The latter produces a sexp with

```
:headers (:Subject "" :From "" :To "" :Reply-To "" :Date "")
```

even when the message has many more headers.

The end result is that `notmuch-message-headers` variable has no effect.