Cannot update installations with useMasterKey

[JeremyPlease]

PR #2739 that fixes issue #2090 caused a regression that prevents installation objects from being updated.

Example:

let installationObj = Parse.Installation.createWithoutData(objectId);
installationObj.set('customField', 'custom value');
installationObj.save(null, {useMasterKey: true});

This returns ParseError { code: 136, message: 'installationId may not be changed in this operation' }

I'll push the failing test case as a WIP PR and try to fix in RestWrite.js. @flovilmart Any quick suggestions or pointers?