getUnion
Create the requested data set, a union of the cached and un-cached data.
    connection.getUnion(set, diff, neededData, cachedData)
  
  Uses getUnion to merge the two sets of data (neededData & cachedData).
Parameters
- set 
{Set}:The parameters of the data set requested.
 - diff 
{Object}:The result of getDiff.
 - neededData 
{ListData}:The data loaded from the base connection.
 - cachedData 
{ListData}:The data loaded from the cacheConnection.