fix: allow an object's Symbols to be observed (vuejs#6704)

Attempting to parseFloat on a Symbol throws the error
`Cannot convert a Symbol value to a string`.

A Symbol can be cast to a string using `.toString()` or `String()` though,
so explicitly casting before parsing resolves the issue, allowing `Vue.set` to
be called on Symbols.

src/shared/util.js

@@ -56,7 +56,7 @@ export function isRegExp (v: any): boolean {
  * Check if val is a valid array index.
  */
 export function isValidArrayIndex (val: any): boolean {
-  const n = parseFloat(val)
+  const n = parseFloat(String(val))
   return n >= 0 && Math.floor(n) === n && isFinite(val)
 }