This is the feature of Asterisk. It is not a bug or a installation issue. Asterisk works as a back-to-back user agent (B2BUA). When you are sending SDP by Asterisk,
Asterisk is changing SDP to prevent P2P. Asterisk is doing so because it needs control over media.
You need a Proxy Server to make a P2P call.
Using Asterisk as a proxy may be possible but it is a popular way and I don't know much about it. When someone works a proxy, it doesn't alter SDP.
Thanks,
Shakeeb