fix(observer): invoke getters on initial observation if setter defined (#7828)

Author: pkaminski

Diff for: src/core/observer/index.js

@@ -147,10 +147,10 @@ export function defineReactive (
 
   // cater for pre-defined getter/setters
   const getter = property && property.get
-  if (!getter && arguments.length === 2) {
+  const setter = property && property.set
+  if ((!getter || setter) && arguments.length === 2) {
     val = obj[key]
   }
-  const setter = property && property.set
 
   let childOb = !shallow && observe(val)
   Object.defineProperty(obj, key, {