Change the handling og the HTTP content length

Change the handling og the HTTP content length

This change should make the indicated HTTP message content length be
as described in RFC 2616.

For requests the content length is 0 if neither Content-Length nor
Transfer-Encoding headers are set. If the content length of a request
is set to 0 the Content-Length header is not sent with a value of
0. For GET and HEAD the content length is set to 0 by default.

For responses content length needs to be set to 0 to indicate an empty
response (unless it is a HEAD response where the content length
indicate the size of the body expected). Otherwise a body is assumed
and if no content length is set chunked transfer encoding is used.

For HTTP 1.0 chunked transfer encoding is not supported. So for HTTP
1.0 requests needs content length if they have a body and responses
without a content length will indicate end of body by closing the
connection.

R=ager@google.com

BUG=dart:6309
Committed: https://code.google.com/p/dart/source/detail?r=14184

[Mads Ager (google), 2012-10-29 11:24:29]

LGTM!