Websocket: Plot data are not sent if message is over 64 kB

At least in "raw" mode, the "count" attribute in Plot subscription is being ignored. Hence,  plot data messages can grow very big. We found out that messages over 64kB (65.536 ASCII characters)   get blocked. Assumption is that the websocket server is limiting the message size. 
See https://forum.fhem.de/index.php/topic,118668.msg1193409.html#new