Revert "Set mergeStrats to data"

This reverts commit 720c24c.

Author: posva

src/vuefire.js

@@ -250,9 +250,9 @@ function install (_Vue) {
   Vue = _Vue
   Vue.mixin(VueFireMixin)
 
-  // use data-like merge strategy
+  // use object-based merge strategy
   var mergeStrats = Vue.config.optionMergeStrategies
-  mergeStrats.firebase = mergeStrats.data
+  mergeStrats.firebase = mergeStrats.methods
 
   // extend instance methods
   Vue.prototype.$bindAsObject = function (key, source, cancelCallback, readyCallback) {

tests/vuefire.spec.js

@@ -1012,30 +1012,4 @@ describe('VueFire', function () {
       })
     })
   })
-
-  describe('mixins', function () {
-    var mixinFn
-    beforeEach(function () {
-      mixinFn = {
-        firebase: function () {
-          return {
-            items: firebaseRef.child('items')
-          }
-        }
-      }
-    })
-
-    it('func/func', function () {
-      var vm = new Vue({
-        firebase: function () {
-          return {
-            item: firebaseRef.child('first')
-          }
-        },
-        mixins: [mixinFn]
-      }).$mount()
-      expect(vm.items).to.not.be.undefined
-      expect(vm.item).to.not.be.undefined
-    })
-  })
 })