Fix response not changing with parallel requests

In this order the @env would not be changed by the "on complete"
callbacks, meaning that the response would not change after parallel
requests finish (e.g. JSON would not be parsed).

This would work

  def on_complete(env)
    env[:body].upcase!
  end

But this wouldn't

  def on_complete(env)
    env[:body] = env[:body].upcase
  end
Merge pull request #378 from janko-m/fix-parallel-parsing

Fix response not changing with parallel requests