Rewrite: Ignore headers if we have proxy_cache_valid set.

We don't want response headers from the endpoints from interfering with proxy_cache_valid. Using proxy_ignore_headers will fix that.

From the proxy_cache_valid documentation:
> Parameters of caching can also be set directly in the response header. This has higher priority than setting of caching time using the directive. The “X-Accel-Expires” header field sets caching time of a response in seconds. The zero value disables caching for a response. If the value starts with the @ prefix, it sets an absolute time in seconds since Epoch, up to which the response may be cached. If the header does not include the “X-Accel-Expires” field, parameters of caching may be set in the header fields “Expires” or “Cache-Control”. If the header includes the “Set-Cookie” field, such a response will not be cached. Processing of one or more of these response header fields can be disabled using the proxy_ignore_headers directive.
Rewrite: Use has_proxy_cache_valid.
Rewrite: Add comments to the proxy_ignore_headers line.
Merge pull request #92 from duckduckgo/jag/ignore-headers

Rewrite: Ignore headers if we have proxy_cache_valid set.