open SockJS connection event document ready state is "interactive"

Some of SockJS transports need `document.body` availability, so there is a test of [`document.readyState`](https://developer.mozilla.org/en-US/docs/Web/API/document.readyState) to be `complete`. But there is one more state means body of document has been loaded and available - `interactive`. 

Why we must test `document.readyState` to be `complete` or `interactive` not only `complete`? 

Just because old versions of Opera browser (10.62 for example) switch `document.readyState` to `interactive` after iframe has been inserted into the DOM, and never switch to `complete` state back. This is only one reason SockJS does not work with Opera <10.70.