Fix lots of requests in single session

Implemented on top of #13 because that adds the test whose failure this PR fixes.

Add `cleanup_headers` method to `ServerHandler` that adds `Connection: Close` response header so that clients don't expect keep alives.

Fixes: #13

Passing Travis CI build: https://travis-ci.org/kevin1024/pytest-httpbin/builds/42608004

```
$ tox
...
  py26: commands succeeded
  py27: commands succeeded
  py34: commands succeeded
  pypy: commands succeeded
  congratulations :)
```