function getKeyAsValueForCacheMap(key) {
          return angular.isObject(key) ? JSON.stringify(key) : key;
        }
@ngdoc function
@name insertGroupHeader
@methodOf  ui.grid.grouping.service:uiGridGroupingService
@description Create a group header row, and link it to the various configuration
items that we use.

Look for the row in the oldGroupingHeaderCache, write the row into the new groupingHeaderCache.

@param {Grid} grid grid object
@param {array} renderableRows the rows that we are processing
@param {number} rowIndex the row we were up to processing
@param {array} processingState the current processing state
@param {number} stateIndex the processing state item that we were on when we triggered a new group header -
i.e. the column that we want to create a header for