phase-handler: pass through NGX_AGAINs from ps_content_handler

When `ngx_http_output_filter` returns `NGX_AGAIN` we need to make sure it gets a chance to keep running.  We're currently passing that `NGX_AGAIN` up to `ps_phase_handler` where it gets lost.  Instead, pass it on to `ngx_http_finalize_request` which will call `ngx_http_set_write_handler`.

Fixes #371