Clean authData of null values on _User update

Adds a step to the RestWrite#execute chain: it cleans the response
authData object of null values.

For example, this:

{"authData": {"anonymous": null}, "updatedAt", ...}

will be transformed to this:

{"updatedAt", ...}

And this:

{"authData": {"anonymous": null, "twitter": ...}, "updatedAt", ...}

will be transformed to this:

{"authData": {"twitter": ...}, "updatedAt", ...}

Fixing this issue will fix anonymous user upgrades from the Android SDK.

src/RestWrite.js

@@ -79,6 +79,8 @@ RestWrite.prototype.execute = function() {
     return this.expandFilesForExistingObjects();
   }).then(() => {
     return this.runDatabaseOperation();
+  }).then(() => {
+    return this.cleanUserAuthData();
   }).then(() => {
     return this.handleFollowup();
   }).then(() => {
@@ -824,5 +826,21 @@ RestWrite.prototype.sanitizedData = function() {
   return Parse._decode(undefined, data);
 }
 
+RestWrite.prototype.cleanUserAuthData = function() {
+  if (this.response && this.response.response && this.className === '_User') {
+    let user = this.response.response;
+    if (user.authData) {
+      Object.keys(user.authData).forEach((provider) => {
+        if (user.authData[provider] === null) {
+          delete user.authData[provider];
+        }
+      });
+      if (Object.keys(user.authData).length == 0) {
+        delete user.authData;
+      }
+    }
+  }
+};
+
 export default RestWrite;
 module.exports = RestWrite;