ProcessHeaders is an overridden procedure that load the header names and values in Headers into their corresponding object properties. RawHeaders is updated to contain the initial data in Headers. Values in Headers are assigned to the corresponding property, and subsequently removed the
TIdHeaderList. ProcessHeaders also determines the values for ContentRangeStart and ContentRangeEnd when Headers contains the 'Range' header.
ProcessHeaders callls the inherited ProcessHeaders method to allow handing of Entity headers, and headers that are stored in ExtraHeaders.