Implement Firestore Query#stream

See https://github.com/googleapis/nodejs-firestore/blob/7da51c95ddce9fbc5973dde8dd3692a315d713a4/src/reference.js#L1701-L1720

Author: squirly

src/firestore-query.js

@@ -2,6 +2,7 @@
 
 var _ = require('./lodash');
 var assert = require('assert');
+var Stream = require('stream');
 var Promise = require('rsvp').Promise;
 var autoId = require('firebase-auto-ids');
 var QuerySnapshot = require('./firestore-query-snapshot');
@@ -108,6 +109,19 @@ MockFirestoreQuery.prototype.get = function () {
   });
 };
 
+MockFirestoreQuery.prototype.stream = function () {
+  var stream = new Stream();
+
+  this.get().then(function (snapshots) {
+    snapshots.forEach(function (snapshot) {
+      stream.emit('data', snapshot);
+    });
+    stream.emit('end');
+  });
+
+  return stream;
+};
+
 MockFirestoreQuery.prototype.where = function (property, operator, value) {
   var query;