Add missing element `xfId` of the styles to fix issue #93.

styles.go

@@ -2122,6 +2122,8 @@ func setCellXfs(style *xlsxStyleSheet, fontID, numFmtID, fillID, borderID int, a
 	style.CellXfs.Count++
 	xf.Alignment = alignment
 	xf.ApplyAlignment = applyAlignment
+	xfID := 0
+	xf.XfID = &xfID
 	style.CellXfs.Xf = append(style.CellXfs.Xf, xf)
 	return style.CellXfs.Count - 1
 }

xmlStyles.go

@@ -211,7 +211,7 @@ type xlsxXf struct {
 	NumFmtID          int             `xml:"numFmtId,attr"`
 	PivotButton       bool            `xml:"pivotButton,attr,omitempty"`
 	QuotePrefix       bool            `xml:"quotePrefix,attr,omitempty"`
-	XfID              *int            `xml:"xfId,attr,omitempty"`
+	XfID              *int            `xml:"xfId,attr"`
 	Alignment         *xlsxAlignment  `xml:"alignment"`
 	Protection        *xlsxProtection `xml:"protection"`
 }