Fix adding Expect: 100-continue header with empty POST body.

If we trying to send a POST request with an empty body (like maximize window command), curl adds additional header "Expect: 100-continue". So, the result request will be like this:
```
POST /wd/hub/session/:sessionId/window/:windowId/maximize HTTP/1.1
Host: 127.0.0.1:4444
Content-Type: application/json;charset=UTF-8
Accept: application/json;charset=UTF-8
Content-Length: 0
Expect: 100-continue

```
It's not a problem for original selenium, but if we use proxy we may receive
```
HTTP/1.1 400 Bad Request
Connection: close
Content-Type: text/plain; charset=utf-8
Content-Length: 0
```

My changes will remove this header from empty request.