feature: Add support for array of requests with delays to pipelined_requests

[gabihodoroaga]

Hi,

I need to be able to add a delay between pipelined requests and currently this is not supported

--- pipelined_requests eval 
["GET /t/1", "GET /t/2", [{value => "GET /t/3", delay_before =>3}]]

will generate a malformed http request

127.0.0.1 - - [03/May/2021:15:10:51 +0200] "GET /t/1 HTTP/1.1" 200 2 "-" "-"
127.0.0.1 - - [03/May/2021:15:10:51 +0200] "GET /t/2 HTTP/1.1" 200 2 "-" "-"
127.0.0.1 - - [03/May/2021:15:10:51 +0200] "ARRAY(0x7fdebfc7d1e8) / HTTP/1.1" 400 157 "-" "-"