Fix hang in wb_update_client with content-length and connection:close

I kind of found this by accident, but I've managed to find a fairly easy repro.

I called WebbyBeginResponse with a positive content-length and with a Connection: close header. That caused wb_update_client to loop forever. 

The fix was just to bail after clearing the WB_ALIVE flag in the WB_CLOSE_AFTER_RESPONSE check, as other code in wb_update_client does. A quick scan of the code found 1 other place that looked suspect - where WEBBY_WS_OP_PING is handled, but I have no repro for that case.