Modify bad CONNECT handling

Changing the way that OkHttp handles CONNECT responses
from proxies.

Before the change:
If a proxy returns a body and some of the body bytes have
already been buffered then an IOException is thrown.
If the body bytes have not been buffered then the handshake will
fail due to the presence of bytes where the Server HELLO is
expected, typically with some kind of SSLHandshakeException.

After the change:
The body bytes are consumed to ensure consistent behavior. The
handshake will then take place.

History:
This is an unusual case that would occasionally cause failures
on Android when a ResponseCache was installed. Android introduced
a patch to prevent CONNECT responses being cached. Since then,
OkHttp has changed the code and probably fixed the issue via
other means. The Android test remained and would sometimes
experience one exception, sometimes another, depending on the
state of the buffer.

If the presence of a body is a possibility it would be nice
to deal with it consistently and deterministically.

The motivation for this change is to make
URLConnectionTest#connectViaHttpProxyToHttpsUsingBadProxyAndHttpResponseCache
behave consistently and in a way that doesn't involve connection
fallback.

The only risk with this change is if a proxy is incorrectly
reporting the content length: this might lead to blocking on
the body read/skip (as OkHttp would probably do elsewhere if a
server mis-reports a content-length). This change may make
connections to "bad" proxies slightly more reliable.