Delete the "Content-Length" header if we aren't copying

Fixes a bug where the Content-Length will mismatch the actual bytes written if we skipped copying the response, so we get a message like this when using curl:

```
curl: (18) transfer closed with 18 bytes remaining to read
```

To replicate:

```
{
	admin off
	debug
}

:8881 {
	reverse_proxy 127.0.0.1:8882 {
		@200 status 200
		handle_response @200 {
			header Foo bar
		}
	}
}

:8882 {
	header Content-Type application/json
	respond `{"hello": "world"}` 200
}
```

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -733,6 +733,13 @@ func (h Handler) finalizeResponse(
 		res.Header.Del(h)
 	}
 
+	// remove the content length if we're not going to be copying
+	// from the response, because otherwise there'll be a mismatch
+	// between bytes written and the advertised length
+	if bodyClosed {
+		res.Header.Del("Content-Length")
+	}
+
 	// apply any response header operations
 	if h.Headers != nil && h.Headers.Response != nil {
 		if h.Headers.Response.Require == nil ||