http: optimize outgoing requests

[brendanashworth]

This commit does some small optimization changes on
lib/_http_outgoing.js. These include switching from while loops to
for loops, moving away from util to typeof checks, and removing
dead code.

Here is a graph of the benchmark
change (duration = 20, ran twice each). Changes aren't very visible in
smaller packets - at 32 bytes, only about 1% more requests can be sent,
but it increases as the packets increase in size; 1024 has 4.7% more
throughput than before.

Here is a graph of the same
benchmark but sorted by encoding type. As it is shown, buffers get the
biggest speed increase, then utf-8, then ASCII.