net.http: Send entire HTTP request header as one write
When opportunistic writes are enabled this reduces the number of
syscalls and TCP packets sent on the wire.
Experiments with TCP Fast Open made this even more obvious.
That table trick probably wasn't as efficient. Lua generates bytecode
for a table with zero array slots and space for two entries in the hash
part, plus code to set [2] and [4]. I didn't verify but I suspect it
would have had to resize the table when setting [1] and [3], although
probably only once. Concatenating the strings directly in Lua is easier
to read and involves no extra table or function call.