Exception when [Parse setApplicationId:clientKey:] is called twice #159
Description
Recently I've found in our codebase that +[Parse setApplicationId:clientKey:] was called twice during app initialization (with same params).
It was obviously mistake, but by the way I've found that in that case SDK throws exceptions, ie:
during call to -[PFCommandURLRequestConstructor _URLRequestHeadersForCommand], dataSource became nil (it's a weak reference to ParseManager).
It'd be nice to have an assertion preventing from changing applicationId/clientKey after it's set already or cancel all ongoing operations started from the old instance of ParseManager.