Add backticks to escape field names.

Add clasp badge to README.

Author: LaughDonor

.github/README.md

@@ -1,6 +1,7 @@
 # Firestore for Google Apps Scripts
 
 [![JavaScript Style Guide](https://img.shields.io/badge/code_style-standard-brightgreen.svg)](https://standardjs.com)
+[![clasp](https://img.shields.io/badge/built%20with-clasp-4285f4.svg)](https://github.com/google/clasp)
 
 ### A Google Apps Script library for accessing Google Cloud Firestore.
 

Query.js

@@ -34,8 +34,10 @@ var FirestoreQuery_ = function (from, callback) {
 
   // @see {@link https://firebase.google.com/docs/firestore/reference/rest/v1/StructuredQuery#FieldReference Field Reference}
   const fieldRef = function (field) {
-    return { 'fieldPath': field }
+    var escapedField = field.split('.').map(function (f) { return '`' + f.replace('`', '\\`') + '`' }).join('.')
+    return { 'fieldPath': escapedField }
   }
+
   const filter = function (field, operator, value) {
     operator = operator.toLowerCase().replace('_', '')