HTTP responses with no body cause other responses to be consumed about dpkt HOT 4 OPEN

Please note that 204 No Content, and others have no body either.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.3

"For response messages, whether or not a message-body is included with
a message is dependent on both the request method and the response
status code (section 6.1.1). All responses to the HEAD request method
MUST NOT include a message-body, even though the presence of entity-
header fields might lead one to believe they do. All 1xx
(informational), 204 (no content), and 304 (not modified) responses
MUST NOT include a message-body. All other responses do include a
message-body, although it MAY be of zero length."

Ok. The current issue is being caused by dpkt reading the rest of the data into 
the body if the response header has a content-type header, which is wrong 
according to the RFC. A simple fix is simply to remove those two lines.

Of course, this doesn't solve the larger issue. Making the response 
intelligently decide whether to parse a body is tricky with dpkt's 
architecture, because the parsing of the body and headers is done in the 
http.Message superclass.

Attached is a patch to fix the issue:
1. Ignore content-type, as it does not say anything about the existence of a 
body.
2. Do not attempt to read body if response status code is 1xx, 204 or 304.
3. Here comes the ugly part: ignore body if parameter head_response=True has 
been passed to Response object. HEAD responses are identical to GET responses 
(including Content-Length), except for the missing body. dpkt can not determine 
automatically whether this is a HEAD response.

Also attached modified dpkt_http_bug2.zip:
- dpkt_bug.py: call dpkt.http.Response(data, head_response=True)
- stream.txt: file ends with '\r\n\r\n' (instead of '\r\n')

Slight improvement: set body consistently to '' (empty string), avoid None.