replace object rest spread with Object.assign for node6 compat. (fix #1245)

Author: LinusBorg

scenarios/index.js

@@ -11,9 +11,9 @@ const isTest = exports.isTest = index !== -1
 const scenario = isTest && require(`./${scenarios[index]}.json`)
 
 exports.addTestAnswers = (metalsmith, options, helpers) => {
-  Object.assign(metalsmith.metadata(), {
-    isNotTest: !isTest,
-    ...(isTest ? scenario : undefined)
-  })
-  // console.log(metalsmith.metadata())
+  Object.assign(
+    metalsmith.metadata(),
+    { isNotTest: !isTest },
+    isTest ? scenario : {}
+  )
 }