Quick fix for #648

[JannisBush]

Allows to specify that a response is belonging to a head request and should not have a body #648

Usage:

import dpkt

head_response = b"HTTP/1.1 200 OK\r\nContent-Length: 10\r\n\r\n"
resp = dpkt.http.Response(head_response, head_response=True)

[coveralls]

Coverage decreased (-0.01%) to 99.808% when pulling b114296 on JannisBush:patch-1 into 97ea45b on kbandla:master.