Prints io directly instead of converting to string

[paulcsmith]

Closes #507

This prints the io directly to the response, rather than making it a
string first. The bigger the response, the more helpful this will be.

Benchmarks of printing directly v. converting to String first:

require "benchmark"

text = <<-TEXT
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua
TEXT

string_text = text * 40_000
io_text = IO::Memory.new(string_text)

Benchmark.ips do |x|
  x.report("string") do
    io = IO::Memory.new
    io.print(io_text.to_s)
  end
  x.report("io") do
    io = IO::Memory.new
    io.print(io_text)
  end
end

Results:

string 942.67  (  1.06ms) (± 1.33%)  12.7MB/op   2.78× slower
    io   2.62k (381.73µs) (± 3.52%)   8.0MB/op        fastest

So far medium/large HTML responses this will shave off about .6ms. Not massive, but a quick and easy win

[paulcsmith]

Also, we could further optimize this, but I wanted to get the easy win first. Maybe we can skip making a new IO to speed it up even more. But we'd have to modify TextResponse and do a few other things so I'm gonna put that off for now