change request body from String to Vec<u8> for binary request body

Trying to do to upload images from the client to the server. 

this is my html code:
```html
    <form action="/upload" method="post" enctype="multipart/form-data">
        <input type=file name=upload_img value="" />
        <input type=submit name="sub" value=" Upload " />
    </form>
```
On uploading images 400 Bad Request always happend, because  src/http/server/request.rs line 381 String::from_utf8(body)

I think request body must be Vec< u8 >. 
What you think about this?

P.S.: sorry for my bad english.