handle chunked parts across multiple reads

fixes https://redmine.lighttpd.net/issues/2554
where Happstack performs multiple writes to send HTTP chunked header,
HTTP chunked body, and CRLF ending HTTP chunked body

supercedes https://github.com/lighttpd/weighttp/pull/8

handle additional chunked case of split CR and LF
reference
  https://github.com/lighttpd/weighttp/pull/9/files#r52358620
  thx stbuehler

detect CR not followed by LF (error)

handle NIL in chunked stream
(should not occur in HTTP headers or chunked header,
 but could occur in chunked body)

x-ref:
  "weighttp closing connection too early(?)"
  https://redmine.lighttpd.net/issues/2554

github: closes #8, closes #9