fix unicode list in respond

here log:

10:16:57.983 [error] Unhandled Error: error:badarg. Stacktrace: [{erlang,iolist_size,[[[22896,32853,26619,39443,34553],<<"\n">>]],[]},{mochicow_request,respond,2,[{file,"src/mochicow_request.erl"},{line,401}]},{boss_web_controller_handle_request,handle_request,3,[{file,"src/boss/boss_web_controller_handle_request.erl"},{line,26}]},{mochicow_upgrade,upgrade,4,[{file,"src/mochicow_upgrade.erl"},{line,72}]},{cowboy_protocol,execute,4,[{file,"src/cowboy_protocol.erl"},{line,529}]}]