Remove NSDataReadingMappedIfSafe when reading attachment content

NSDataReadingMappedIfSafe option can keep the file descriptors of the attachments open if the client application keeps holding the NSData object returned by CBLAttachment.content.

For a very big attachment, alternatively, users can call contentURL and read the NSData with the NSDataReadingMappedIfSafe option themselves.

#649

Source/API/CBLAttachment.m

@@ -118,7 +118,7 @@ - (NSData*) content {
             return _body;
         else if ([_body isKindOfClass: [NSURL class]] && [_body isFileURL]) {
             return [NSData dataWithContentsOfURL: _body
-                                         options: NSDataReadingMappedIfSafe | NSDataReadingUncached
+                                         options: NSDataReadingUncached
                                            error: nil];
         }
         return nil;