Preserve initial `from` and `to` attributes in BOSH session state

The XEP[1] states:

    The <body/> element of the first request SHOULD possess the following
    attributes (they SHOULD NOT be included in any other requests [...]):

    - 'to' -- This attribute specifies the target domain of the first stream.

    [...]

    The <body/> element of the first request MAY also possess
    a 'from' attribute.

This fixes stream restart problem reported in #306.

[1]: http://xmpp.org/extensions/xep-0124.html