Flush bad request and disconnect responses

[diyan]

Without flushing we will have empty response.

For example Nginx will return HTTP 502 Bad Gateway instead of correct response in such cases.

[diyan]

Also I did a bugfix for JSONPolling transport.

Please let me know if you would like me create separate pull request.

Commits with "JSONPolling" prefix fixes a following warning that appears in browsers:

Resource interpreted as Script but transferred with MIME type text/plain: "https://example.com/socket.io/1/jsonp-polling/580139618792?t=1410183159408&i=0".