requestEnd: Fix detection on when response is finished

flaska.mjs

@@ -467,7 +467,7 @@ export class Flaska {
     if (err) {
       this._onerror(err, ctx)
     }
-    if (ctx.req.complete) {
+    if (ctx.res.writableEnded) {
       return
     }
 

test/flaska.out.test.mjs

@@ -198,6 +198,17 @@ t.describe('#requestEnd()', function() {
     flaska.requestEnd(null, ctx)
   })
   
+  t.test('should return immediately if req is finished', function() {
+    let onFinish = function() {
+      throw new Error('should not be called')
+    }
+    const ctx = createCtx({ }, onFinish)
+    ctx.res.writableEnded = true
+
+    let flaska = new Flaska({}, fakerHttp, fakeStream)
+    flaska.requestEnd(null, ctx)
+  })
+  
   const emptyStatuses = [204, 205, 304]
 
   emptyStatuses.forEach(function(status) {