Fix a Ruby warning in File::WriteOptions

The warning was:

```
lib/sugar_utils/file/write_options.rb:42: warning: instance variable @existing_owner not initialized
```

I have added some explicit initialization for the instance variables.

lib/sugar_utils/file/write_options.rb

@@ -7,8 +7,10 @@ class WriteOptions
       # @parma filename [String]
       # @param options [Hash]
       def initialize(filename, options)
-        @filename = filename
-        @options  = options
+        @filename       = filename
+        @options        = options
+        @existing_owner = nil
+        @existing_group = nil
 
         return unless filename && ::File.exist?(filename)