removed hardcoded response content-type definition in the EventsPushHandler

removed hardcoded response content-type definition (application/javascript) because this broke streaming for internet explorer (asking to download eventbus.js instead of reading the streamed payload).

no problems with websockets in ff/chrome and no problems with streaming in ie/ff/chrome could be observed without that line. if i understood it correctly, atmosphere handles the browser specific content-type stuff anyways.