ngx_pagespeed: respect ModPagespeed=off
ngx_pagespeed: respect ModPagespeed=off

One of the things you can do with headers or query params is turn off mod
pagespeed:

    curl -D- 'http://localhost:8050/mod_pagespeed_example/combine_css.html'
    curl -D- 'http://localhost:8050/mod_pagespeed_example/combine_css.html\
      ?ModPagespeed=off'

We weren't respecting this fully before, still removing the etag, dropping the
content length, etc.

merge of pull request #28

digideskio/ngx_pagespeed,wanrui/ngx_pagespeed,0x0mar/ngx_pagespeed,pagespeed/ngx_pagespeed,wanrui/ngx_pagespeed,digideskio/ngx_pagespeed,0x0mar/ngx_pagespeed,pagespeed/ngx_pagespeed