SPDY: refactor SpdyHttpDecoder to allow subclasses to access messageMap
Fix for #1313 and #1332

Add TEXTPLAIN in HttpHeaders
Remove all coding/decoding Attribute using URL scheme for name and
attribute
Add text/plain to contant-type
No need to pass the next BufType to the constructor
Fix examples
Try to reproduce #1335 without luck
[#1293] Fix handling of chunked requests in HttpPostRequestEncoder and the multipart examples