refactor http.read_http_body

This PR refactors read_http_body to work according to the [spec](http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-16#section-3.3). This has been neccessary as the current version omits response bodies if no content-length header is set.

In particular, <code>http://spiegel.ivwbox.de/2004/01/survey.js</code> did not work as of 0.9.2.

Theses changes affect pathod and mitmproxy as well.