Improved HTTP headers parsing

[rschildmeijer]

selam pointed out: https://github.com/rschildmeijer/deft/commit/6601460362c8a3e27984a17c296c7d6d615cba6e#commitcomment-399036

short version: "The root cause is the naive parsing in o.d.web.http.HttpRequest#of. Should probably do something similar to the PartialHttpRequest solution. (Its dangerous to assume that the first read from the channel will contain all headers.)"