statistics: /ngx_pagespeed_statistics was broken again

If we get a request for `/ngx_pagespeed_statistics` we don't create a `ps_request_context`.  We create the content properly, but then in the header filter we assume that "ctx is NULL iff this is not a resource fetch" and so conclude that we're not a resource fetch.  Then we call `CreateRequestContext()` with `resource_fetch=false` and so it doesn't check to see if it's statistics.

Here I fix this by pulling the check for statistics outside of the resource-fetch-only part of `CreateRequestContext()`.