Return unclosed body to fix localhost redirects

[mikesep]

Use case scenario:
I'm testing a server locally that serves a 302 redirect. If I curl localhost:<port> through pacproxy, curl gives me a warning that the Content-Length header doesn't match the actual length of content in the response body.

Note: Redirects from remote servers seem to work fine. I think curl has special logic to slightly shortcut the proxy when you make a localhost request -- it sends an absolute URL rather than a CONNECT request.

From the documentation for net/http.Client:

If CheckRedirect returns an error, the Client's Get method returns both the previous Response (with its Body closed) and CheckRedirect's error (wrapped in a url.Error) instead of issuing the Request req. As a special case, if CheckRedirect returns ErrUseLastResponse, then the most recent response is returned with its body unclosed, along with a nil error.

Since the response's Body is closed, no content is relayed, but the Content-Length header still makes it through, leading to the mismatch.

ErrUseLastResponse didn't exist when pacproxy's CheckRedirect function was originally written. :)

[williambailey]

Thanks for the PR. I'll try to get this in a new release over the next few days.