feat(v-text/v-html): Add the ability to render custom toString()

vuejs#8093

Author: mathieutu

src/shared/util.js

@@ -77,7 +77,7 @@ export function isValidArrayIndex (val: any): boolean {
 export function toString (val: any): string {
   return val == null
     ? ''
-    : typeof val === 'object'
+    : typeof val === 'object' && (!isPlainObject(val) || val.toString() === _toString.call(val))
       ? JSON.stringify(val, null, 2)
       : String(val)
 }