Set mergeStrats to data

Allow merging firebase options when defined as functions

src/vuefire.js

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

tests/vuefire.spec.js

@@ -1012,4 +1012,30 @@ 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
+    })
+  })
 })