update nginx_test_helper gem
removed push_stream_keepalive directive, let Nginx decide when to use keepalive and how many requests accept using keepalive_* directives
removed push_stream_content_type directive, use default_type Nginx directive to set the value, except on JSONP and Event Source modes
renamed broadcast feature to wildcard, more adequate with the way it works
changed push_stream_store_messages directive to make possible set it inside an if block