[assets] confirm a weak but matching ETag

When handling compression at the proxy server level, the client receives a weak ETag.
Weak ETags are prefixed with `W/`, e.g. `W/"2.2.0"`.
Upon cache validation we should take care of these too.

lib/index.js

@@ -358,10 +358,11 @@ Server.prototype.serve = function(req, res){
   // Per the standard, ETags must be quoted:
   // https://tools.ietf.org/html/rfc7232#section-2.3
   var expectedEtag = '"' + clientVersion + '"';
+  var weakEtag = 'W/' + expectedEtag;
 
   var etag = req.headers['if-none-match'];
   if (etag) {
-    if (expectedEtag == etag) {
+    if (expectedEtag == etag || weakEtag == etag) {
       debug('serve client 304');
       res.writeHead(304);
       res.end();