KAZOO-3157

actually, we should return binary instead of iolist from render, gen_smtp does accept binaries in body and headers. i didn't took that path because notify app has modules with internal functions using the result of render and their specs refer a iolist. i was trying to make a minimal change that would leave the system working and then take it to discussion about converting to binary the iolist result of render and triple check the modules that use it. since we're going to make changes in teletype (adding modules), we can make the change in that timeframe.