Remove database field from REST API request body. (#2188)

* Remove database field from REST API request body.

* [AUTOMATED]: Prettier Code Styling

* Fix type and add comments about the emulator.

* [AUTOMATED]: Prettier Code Styling

Author: yuchenshi

packages/firestore/src/platform_browser/webchannel_connection.ts

@@ -38,6 +38,7 @@ import { StreamBridge } from '../remote/stream_bridge';
 import { assert, fail } from '../util/assert';
 import { Code, FirestoreError } from '../util/error';
 import * as log from '../util/log';
+import { Indexable } from '../util/misc';
 import { Rejecter, Resolver } from '../util/promise';
 import { StringMap } from '../util/types';
 
@@ -172,7 +173,13 @@ export class WebChannelConnection implements Connection {
         }
       });
 
-      const requestString = JSON.stringify(request);
+      // The database field is already encoded in URL. Specifying it again in
+      // the body is not necessary in production, and will cause duplicate field
+      // errors in the Firestore Emulator. Let's remove it.
+      const jsonObj = ({ ...request } as unknown) as Indexable;
+      delete jsonObj.database;
+
+      const requestString = JSON.stringify(jsonObj);
       log.debug(LOG_TAG, 'XHR sending: ', url + ' ' + requestString);
       // Content-Type: text/plain will avoid preflight requests which might
       // mess with CORS and redirects by proxies. If we add custom headers