Include IdleTimeoutFilter conditionally based on X-Accept-Commands in wseb

[jitsni]

Gateway's IdleTimeoutFilter sends PONGs in writer is idle. Some older clients (like .NET 4.0) don't understand PONGs. The proposal is to use X-Accept-Commands: pong to include IdleTimeoutFilter and thus it becomes conditional.

Newer clients need to send X-Accept-Commands:pong and process PONGs. Older clients don't get PONGs (they rely on NOOP command) as they don't send X-Accept-Commands:pong

We need to update x-kaazing-idle-timeout specification accordingly and add some specification tests for wseb.