Fixed scan with skip (#2357)

* Fixed bad test for scan

The previous test didn't catch the fact that the accumulator had been
broke, it's value became the special value `SKIP`.

* Fixed Stream.scan() to accept SKIP value

* Update stream/stream.js

Dropped unnecessary ternary as suggested by @isiahmeadows

Co-Authored-By: gamb <adam@gamb.co>

stream/stream.js

@@ -113,8 +113,9 @@ function merge(streams) {
 
 function scan(fn, acc, origin) {
 	var stream = origin.map(function(v) {
-		acc = fn(acc, v)
-		return acc
+		var next = fn(acc, v)
+		if (next !== Stream.SKIP) acc = next
+		return next
 	})
 	stream(acc)
 	return stream