Go's HTTP file server (FileServer & ServeContent) only supports a single byte range.

We need to support multiple byte ranges and generate the multipart response.  For
example:

$ telnet apache.org 80
GET / HTTP/1.1
Host: apache.org
Range: bytes=1-10,20-25

HTTP/1.1 206 Partial Content
Date: Thu, 28 Jun 2012 20:45:12 GMT
Server: Apache/2.4.1 (Unix) OpenSSL/1.0.0g
Last-Modified: Thu, 28 Jun 2012 20:12:15 GMT
ETag: "8f13-4c38df2a86c8a"
Accept-Ranges: bytes
Content-Length: 235
Vary: Accept-Encoding
Cache-Control: max-age=3600
Expires: Thu, 28 Jun 2012 21:45:12 GMT
Content-Type: multipart/byteranges; boundary=176b487255f68cc5


--176b487255f68cc5
Content-type: text/html; charset=utf-8
Content-range: bytes 1-10/36627

!DOCTYPE H
--176b487255f68cc5
Content-type: text/html; charset=utf-8
Content-range: bytes 20-25/36627

C "-//
--176b487255f68cc5--