Fix Heartbeat Response V1+ field ordering

[jessekempf]

https://kafka.apache.org/11/protocol.html#The_Messages_Heartbeat

The V0 heartbeat response looks like:

Heartbeat Response (Version: 0) => error_code 
  error_code => INT16

The V1 heartbeat response looks like:

Heartbeat Response (Version: 1) => throttle_time_ms error_code 
  throttle_time_ms => INT32
  error_code => INT16

This change makes it so that the V1 heartbeat responses no longer have the error code mistakenly reported as a time.Duration.