Allow http_parser to decide if there is still body to read

[Bonias]

It mostly fixes problem when connection which uses "Transfer-Encoding: chunked" header is closed to early, before the terminating chunk is received. In such case "http" returned partial body instead of raising error.

---

Here is how the problem can be reproduced:

Run this app (rackup -p 3000 -E none -s puma):

$ cat config.ru 
class App
  def call(env)
    [200, {}, self]
  end

  def each
    yield 'one'
    sleep 1
    yield 'two'
    sleep 2
    yield 'three'
    sleep 3
    yield 'four'
    sleep 4
    yield 'five'
    sleep 5
    yield 'six'
  end
end

use Rack::Chunked
run App.new

Use irb console to make http request to the app and then kill the app (in the middle of the request):

2.2.3 :001 > require 'http'
 => true 
2.2.3 :002 > HTTP.get('http://localhost:3000/').body.to_s
 => "onetwo" 

---

All tests are passing so my fix seems to be fine, but if you think otherwise let me know and I'll try change it.

[woutdegeyter]

@ixti I am wondering if you would happen to have any news on this PR?
We have encountered multiple issues due to this already

[ixti]

Thanks! Merged as 72723df

[Bonias]

@ixti Sorry to bother you, is there any chance for release a new version in the near future?

[ixti]

@Bonias thanks for reminder :D I will handle that during this week (or over this weekend).