Move setting of $original_upload_dir in S3_Uploads to the constructor

[fklein-lu]

The S3_Uploads class contains a $original_upload_dir member variable. Its purpose is to store the array containing the data related to the original (i.e. on the local file system) upload directory.

This variable is set twice in the class, once in the filter_upload_dir() method (to the value passed by the filter), and once in get_original_upload_dir() (to the value returned by wp_upload_dir()).

This is ambiguous, I think we should set the value once in the constructor, to the value returned by wp_upload_dir(), and this before any filtering by the plugin could happen.

[joehoyle]

wp_upload_dir is a slow function, so I'd rather not invoke it in the constructor, but yes there may be a better way!