static-handler: refactor to use write_handler_response

This fixes sending out the weak ETag for ngx_pagespeed_static/js_defer.hash.js,
and simplifies things somewhat for this code path.
It also adds a test for it.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/434
Merge pull request #435 from pagespeed/oschaaf-refactor-static-handler

static-handler: refactor to use write_handler_response