fix FileBodyGenerator regionSeek

fix: the argument regionSeek takes no effect in FileBodyGenertor
Hello, I just wrote a simple test for posting a file. 
``` java
public void testUpload() throws Exception {
    AsyncHttpClient client = new AsyncHttpClient();
    Response response = client.preparePost("http://httpbin.org/post")
            .setBody(new FileBodyGenerator(new File("./data/text"), 0, 1))
            .execute().get();
    System.out.println(response.getResponseBody());
}
```

http://httpbin.org/ is a site for testing http request. /post will return the raw post data. The file "text" contains string "0123456789". 

When I run the test above, the "data" field returns "0". The result is correct. However, when I set the argument regionSeek to 1, the "data" field also returns "0", which "1" expected. 

It seems that the argument position in method tranferTo is based on the very beginning of the file, seek would not change the result of transferTo. So I made this little patch, then the test passed. 

I don't know if there is something wrong with my code, I will be appreciated if you could point out the problem, thanks.