This closes #986, fix set data validation drop list failed in some cases

Update documentation for `GetCellValue` and simplify code

cell.go

@@ -34,8 +34,8 @@ const (
 // GetCellValue provides a function to get formatted value from cell by given
 // worksheet name and axis in spreadsheet file. If it is possible to apply a
 // format to the cell value, it will do so, if not then an error will be
-// returned, along with the raw value of the cell. All cells value will be
-// same in a merged range.
+// returned, along with the raw value of the cell. All cells' values will be
+// the same in a merged range.
 func (f *File) GetCellValue(sheet, axis string) (string, error) {
 	return f.getCellStringFunc(sheet, axis, func(x *xlsxWorksheet, c *xlsxC) (string, bool, error) {
 		val, err := c.getValueFrom(f, f.sharedStringsReader())

datavalidation.go

@@ -154,7 +154,7 @@ func (dd *DataValidation) SetRange(f1, f2 float64, t DataValidationType, o DataV
 //
 func (dd *DataValidation) SetSqrefDropList(sqref string, isCurrentSheet bool) error {
 	if isCurrentSheet {
-		dd.Formula1 = sqref
+		dd.Formula1 = fmt.Sprintf("<formula1>%s</formula1>", sqref)
 		dd.Type = convDataValidationType(typeList)
 		return nil
 	}

picture.go

@@ -622,7 +622,7 @@ func (f *File) drawingsWriter() {
 }
 
 // drawingResize calculate the height and width after resizing.
-func (f *File) drawingResize(sheet string, cell string, width, height float64, formatSet *formatPicture) (w, h, c, r int, err error) {
+func (f *File) drawingResize(sheet, cell string, width, height float64, formatSet *formatPicture) (w, h, c, r int, err error) {
 	var mergeCells []MergeCell
 	mergeCells, err = f.GetMergeCells(sheet)
 	if err != nil {