Avoid unnecessary yield if buffer has not fully accumulated yet

This reduces the number of yields from WebSocket to the stream parser by
accumulating self.reading_buffer_size worth of bytes before yielding.

ws4py/websocket.py

@@ -387,12 +387,19 @@ def once(self):
             logger.debug("WebSocket is already terminated")
             return False
         try:
-            if not self.buf:
-                b = self.sock.recv(self.reading_buffer_size)
+            wanted = self.reading_buffer_size - len(self.buf)
+
+            if wanted > 0:
+                b = self.sock.recv(wanted)
+
                 if self._is_secure:
                     b += self._get_from_pending()
-                if not b:
+
+                if not b:       # 0-length read. flush buffer and exit
+                    if self.buf:
+                        self.process(self.buf)
                     return False
+
                 self.buf += b
         except (socket.error, OSError, pyOpenSSLError) as e:
             if hasattr(e, "errno") and e.errno == errno.EINTR: