Strip Accept-Ranges with IPRO

[jeffkaufman]

When serving with IPRO we should strip Accept-Ranges because we don't (and can't) support range requests.

Here's modpagespeed.com advertising range support on an ipro resource:

$ headers modpagespeed.com/images/Puzzle.jpg | grep ^Accept-Ranges:
Accept-Ranges: bytes

But it doesn't support them:

 $ headers -H 'Range: bytes=2-4' modpagespeed.com/images/Puzzle.jpg
HTTP/1.1 200 OK
...

Here is how Apache handles range requests with PageSpeed off, supporting them fully:

$ headers -H 'Range: bytes=2-4' modpagespeed.com/images/Puzzle.jpg?PageSpeed=off
HTTP/1.1 206 Partial Content
Date: Tue, 23 Aug 2016 13:21:55 GMT
Server: Apache/2.4.7 (Ubuntu)
Last-Modified: Fri, 10 Jul 2015 19:19:44 GMT
Accept-Ranges: bytes
Content-Length: 3
Cache-control: public, max-age=600
Content-Range: bytes 2-4/241260
Content-Type: image/jpeg