Only fetch once when calling `+ (NSFetchedResultsController *) MR_fetchAllSortedBy:(NSString *)sortTerm ascending:(BOOL)ascending withPredicate:(NSPredicate *)searchTerm groupBy:(NSString *)groupingKeyPath;`
Merge pull request #458 from tonyarnold/hotfix/fix-double-fetch

Only fetch once when calling `+ (NSFetchedResultsController *) MR_fetchAllSortedBy...`