Add test_httpbin_accepts_lots_of_get_requests_in_single_session

This test is actually failing for me at least part of the time on py34.

This was inspired by a bug in httpie:
https://github.com/jakubroztocil/httpie/issues/278#issuecomment-65005675
Add cleanup_headers to ServerHandler

that adds `Connection: Close` response header so that clients don't
expect keep alives.

Fixes: #13
Merge pull request #16 from msabramo/fix_lots_of_requests_in_single_session

Fix lots of requests in single session