max-buffer-size: use ngx_pagesize

Other modules seem to use ngx_pagesize as the default buffer size:
http://lxr.evanmiller.org/http/source/http/modules/ngx_http_gunzip_filter_module.c#L661
Merge pull request #474 from pagespeed/oschaaf-max-buffer-size

max-buffer-size: use ngx_pagesize