Updated evhttp-sharp

Before:
Running 25s test @ http://127.0.0.1:8083/
  10 threads and 600 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     6.38ms    5.51ms  59.43ms   79.82%
    Req/Sec    10.80k     3.90k   29.20k    66.14%
  2469415 requests in 24.99s, 329.70MB read
Requests/sec:  98803.25
Transfer/sec:     13.19MB


After:
Running 25s test @ http://127.0.0.1:8083/
  10 threads and 600 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     8.06ms   12.65ms  81.59ms   90.45%
    Req/Sec    12.36k     4.94k   26.73k    71.89%
  2983443 requests in 24.99s, 398.33MB read
Requests/sec: 119391.93
Transfer/sec:     15.94MB