Add facebook.har test case

facebook.com.har in http_samples covers a corner case where referenced
static header entry evicts implicitly emitted header according to
nghttp2 HPACK encoding algorithm.  Recently, Firefox fixes this bug
https://bugzilla.mozilla.org/show_bug.cgi?id=1019577 Current data sets
do not contain this test.
Add story_31.json of node-http2-hpack
Merge pull request #18 from http2jp/add-facebook-har-test-case

Add facebook.har test case