feat: support firebase@5

* index.js: docChanges is now a method

The Firebase JS SDK introduced some breaking changes in 5.0.0:

firebase/firebase-js-sdk@0997040

Fixes #183.

* package.json: Update firebase peerDependency to 5.0.0

* Update package.json

* Update index.js

Author: zhaofengli

src/index.js

@@ -119,12 +119,14 @@ function bindCollection ({
     }
   }
 
-  const unbind = collection.onSnapshot(({ docChanges }) => {
+  const unbind = collection.onSnapshot((ref) => {
     // console.log('pending', metadata.hasPendingWrites)
     // docs.forEach(d => console.log('doc', d, '\n', 'data', d.data()))
     // NOTE this will only be triggered once and it will be with all the documents
     // from the query appearing as added
     // (https://firebase.google.com/docs/firestore/query-data/listen#view_changes_between_snapshots)
+    const docChanges = typeof ref.docChanges === 'function' ? ref.docChanges() : ref.docChanges
+
     if (!isResolved && docChanges.length) {
       // isResolved is only meant to make sure we do the check only once
       isResolved = true

test/helpers/mock.js

@@ -142,7 +142,7 @@ class CollectionReference extends callbacksAndErrors {
     setTimeout(() => {
       // Object.keys(this.data).map((k, i) => console.log(k, 'at', i, this.data[k].data))
       cb({
-        docChanges: Object.keys(this.data).map((id, newIndex) => ({
+        docChanges: () => Object.keys(this.data).map((id, newIndex) => ({
           type: 'added',
           doc: new DocumentSnapshot(null, new Key(id), this.data[id].data),
           newIndex,
@@ -162,7 +162,7 @@ class CollectionReference extends callbacksAndErrors {
       index: Object.keys(this.data).length
     })
     this._callCallbacks({
-      docChanges: [
+      docChanges: () => [
         {
           type: 'added',
           doc: new DocumentSnapshot(null, id, data),
@@ -194,7 +194,7 @@ class CollectionReference extends callbacksAndErrors {
     const ref = this.data[id.v]
     delete this.data[id.v]
     this._callCallbacks({
-      docChanges: [
+      docChanges: () => [
         {
           doc: new DocumentSnapshot(null, id, ref.data),
           type: 'removed'
@@ -213,7 +213,7 @@ class CollectionReference extends callbacksAndErrors {
       type = 'added'
     }
     this._callCallbacks({
-      docChanges: [
+      docChanges: () => [
         {
           type,
           doc: new DocumentSnapshot(null, id, data),