fix: Merge strategy for provide option

Fixes vuejs#6008

Author: znck

src/core/util/options.js

@@ -183,6 +183,7 @@ strats.watch = function (parentVal: ?Object, childVal: ?Object): ?Object {
 strats.props =
 strats.methods =
 strats.inject =
+strats.provide =
 strats.computed = function (parentVal: ?Object, childVal: ?Object): ?Object {
   if (!childVal) return Object.create(parentVal || null)
   if (!parentVal) return childVal