veb: add ctx.no_content() + prevent content-type being set if empty

[louis77]

veb currently lacks a convenient way to return a 204 No Content response without a body and no content-type header.

This PR adds such a method and prevents ctx.send_response_to_client to always set a content-type header, even if it is set to an empty string.

// send a 204 No Content response without body and content-type
pub fn (mut ctx Context) no_content() Result {
	ctx.res.set_status(.no_content)
	return ctx.send_response_to_client('', '')
}

Using curl, the response looks like this:

> GET / HTTP/1.1
> Host: localhost:8888
> User-Agent: curl/8.7.1
> Accept: */*
> 
* Request completely sent off
< HTTP/1.1 204 No Content
< Server: veb