From @aaron comments in code review:

This seems a bit odd in that $headers for headers to send and $optHeaders is for client request headers. Would this be cleaner as two methods?

Otherwise it seems like only one value of the tuple returned would be useful at once, usually.

Another thing that is possible is to make $headers accept name => value maps as well. Integer keys would imply the old way and string ones the name/value way.

$optHeaders could also be lowercased within stream().