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
[#1293] Fix IllegalBufferAccessException in HttpPostRequestDecoder

* Also let HttpPostRequestDecoder extends Iterator and let its Exceptiosn extend DecoderException
[#1340] Log SslExceptions thrown by SslEngine.closeInbound() with debug level
[#1340] use full url in comment