Add method to get the meta value on BeforeDataFetch when changing order

[walisc]

Currently the BeforeDataFetch method allows you to modify the query. When using order by you need to use the correct meta_value for the field, which is {postTypeId}_{fieldId}_value_key. You need to create a method that get this correct value. May post->GetFieldDbKey().

Also the documentation is wrong on this, Need to update it

Also linked to #13