Add a POST benchmark. Notably to show all the garbage from the body buffer.

For reference, where we're currently at:

$ go test -v -run=^$ -bench=.  -benchtime=2s
PASS
BenchmarkServerGets   5000  527516 ns/op    1559 B/op   48 allocs/op
BenchmarkServerPosts  3000  747704 ns/op   69146 B/op   66 allocs/op
1 file changed