Fix 3 unit tests failing

Fixed tests :
TestCases.HSSF.Record.TestPaletteRecord.TestDefaultPalette
TestCases.HSSF.UserModel.TestHSSFPalette.TestBadIndexes
TestCases.HSSF.UserModel.TestHSSFPalette.TestGnumericStrings

Root cause : Reflection GetField is case sensitive -> index2 field not
found but Index2 is found.