ssi: fix handling of server side includes

Make us live in peace with nginx's SSI module:

- Moved our module so that we we see the content that the ssi module produces as
  a whole. (right before the gzip module does its work).
- Made the header and content filters bail out for subrequests, so we don't
  attempt to rewrite the sub requests that the ssi module fires.
- Changed NGX_HTTP_SSI_BUFFERED to use an unused flag, so I could rule out that
  the SSI module got confused by ngx_pagespeed manipulating that flag as well.
- Handled the case in ps_update() where CollectAccumulatedWrites gives us a null
  cl

Squash-merge of Otto's #242.