DownstreamCachePurgeLocationPrefix removes cache headers for .pagespeed. resources

[oschaaf]

As reported at ngx-pagespeed-discuss:
Adding this line to nginx.conf makes the cache headers disappear for .pagespeed. resources:

pagespeed DownstreamCachePurgeLocationPrefix http://localhost:80/;

According to @anupamadutta:

"Regarding Expires header, I am wondering if https://github.com/pagespeed/ngx_pagespeed/pull/555/files could have anything to do with this. This was intended to be applied for HTML only, but I don't know if it happened to somehow get rid of Expires (and other headers) for other resources as well. Could you take a look?"

[oschaaf]

Confirmed fixed per email
Merged via #653
Closing