spreadsheet: don't allow extra content on Workbook

Fixes unidoc#107

schema/soo/sml/CT_ConditionalFormatting.go

@@ -20,7 +20,7 @@ import (
 type CT_ConditionalFormatting struct {
 	// PivotTable Conditional Formatting
 	PivotAttr *bool
-	// Sequence of Refernces
+	// Sequence of References
 	SqrefAttr *ST_Sqref
 	// Conditional Formatting Rule
 	CfRule []*CT_CfRule

schema/soo/sml/CT_Workbook.go

@@ -58,7 +58,6 @@ type CT_Workbook struct {
 	WebPublishObjects *CT_WebPublishObjects
 	// Future Feature Data Storage Area
 	ExtLst *CT_ExtensionList
-	Extra  []gooxml.Any
 }
 
 func NewCT_Workbook() *CT_Workbook {
@@ -152,11 +151,6 @@ func (m *CT_Workbook) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
 		seextLst := xml.StartElement{Name: xml.Name{Local: "ma:extLst"}}
 		e.EncodeElement(m.ExtLst, seextLst)
 	}
-	for _, any := range m.Extra {
-		if err := any.MarshalXML(e, xml.StartElement{}); err != nil {
-			return err
-		}
-	}
 	e.EncodeToken(xml.EndElement{Name: start.Name})
 	return nil
 }
@@ -274,11 +268,10 @@ lCT_Workbook:
 					return err
 				}
 			default:
-				any := &gooxml.XSDAny{}
-				if err := d.DecodeElement(any, &el); err != nil {
+				gooxml.Log("skipping unsupported element on CT_Workbook %v", el.Name)
+				if err := d.Skip(); err != nil {
 					return err
 				}
-				m.Extra = append(m.Extra, any)
 			}
 		case xml.EndElement:
 			break lCT_Workbook

schema/soo/sml/Workbook.go

@@ -149,11 +149,10 @@ lWorkbook:
 					return err
 				}
 			default:
-				any := &gooxml.XSDAny{}
-				if err := d.DecodeElement(any, &el); err != nil {
+				gooxml.Log("skipping unsupported element on Workbook %v", el.Name)
+				if err := d.Skip(); err != nil {
 					return err
 				}
-				m.Extra = append(m.Extra, any)
 			}
 		case xml.EndElement:
 			break lWorkbook

xsdany.go

@@ -39,6 +39,7 @@ var wellKnownSchemas = map[string]string{
 	"wpi":     "http://schemas.microsoft.com/office/word/2010/wordprocessingInk",
 	"wps":     "http://schemas.microsoft.com/office/word/2010/wordprocessingShape",
 	"xsi":     "http://www.w3.org/2001/XMLSchema-instance",
+	"x15ac":   "http://schemas.microsoft.com/office/spreadsheetml/2010/11/ac",
 }
 
 var wellKnownSchemasInv = func() map[string]string {