Report the "from" attribute of the opening <open /> tag

[sloonz]

XEP-0115 states :

When a connected client or peer server sends a service discovery information request to determine the entity capabilities of a server that advertises capabilities via the stream feature, the requesting entity MUST send the disco#info request to the server's JID as provided in the 'from' attribute of the response stream header (the 'from' attribute was recommended by RFC 3920 [22] and is required by RFC 6120 [11]).

However, strophe.js just silently discards the "from" attribute, so I can’t reuse it for the disco#info request.