log RebalanceInProgress in ConsumerGroup#heartbeat as warning

Resolves #847

Author: marcoroth

lib/kafka/consumer_group.rb

@@ -113,9 +113,12 @@ def heartbeat
 
         Protocol.handle_error(response.error_code)
       end
-    rescue ConnectionError, UnknownMemberId, RebalanceInProgress, IllegalGeneration => e
+    rescue ConnectionError, UnknownMemberId, IllegalGeneration => e
       @logger.error "Error sending heartbeat: #{e}"
       raise HeartbeatError, e
+    rescue RebalanceInProgress => e
+      @logger.warn "Error sending heartbeat: #{e}"
+      raise HeartbeatError, e
     rescue NotCoordinatorForGroup
       @logger.error "Failed to find coordinator for group `#{@group_id}`; retrying..."
       sleep 1