http: Properly reject requests with Transfer-Encoding set

Asterisk does not support any of the transfer encodings specified in
HTTP/1.1, other than the default "identity" encoding.

According to RFC 2616:

   A server which receives an entity-body with a transfer-coding it does
   not understand SHOULD return 501 (Unimplemented), and close the
   connection. A server MUST NOT send transfer-codings to an HTTP/1.0
   client.

This patch adds the 501 Unimplemented response, instead of the hard work
of actually implementing other recordings.

This behavior is especially problematic for Node.js clients, which use
chunked encoding by default.

(closes issue ASTERISK-22486)
Review: https://reviewboard.asterisk.org/r/3092/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@404565 f38db490-d61c-443f-a65b-d21fe96a405b