Ensure that response was received from expected origin

Without this check, it's possible for another window or frame
loaded by the caller of WinChan.open() to send a message that
is interpreted as the WinChan response.

https://bugzilla.mozilla.org/show_bug.cgi?id=868967
Merge pull request #31 from fmarier/bug868967

Ensure that response was received from expected origin