Fixed #823 by adding a hash implementation for DDFileLogger - same as isEqual, it only considers the filePath

Classes/DDFileLogger.m

@@ -1437,6 +1437,10 @@ static int exception_count = 0;
     return NO;
 }
 
+-(NSUInteger)hash {
+    return [filePath hash];
+}
+
 - (NSComparisonResult)reverseCompareByCreationDate:(DDLogFileInfo *)another {
     NSDate *us = [self creationDate];
     NSDate *them = [another creationDate];