Add further handling for when a CONNECT incorrectly returns a body.

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 an Android test
equivalent to the one added here 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 may make connections
to "bad" proxies slightly more reliable.

Bug: 6754912
Bug: 18575971

(cherry picked from commit 007b88ad)

Change-Id: Icf9e17a26dd2b1c4024935dfae3e87b2654a220e