Allow setting of attributes

Also document how to create an XML file without reading from storage.

Readme.md

@@ -4,7 +4,7 @@
 
 Go XML is a DOM based XML representation for Go. The entire XML
 file is read into a set of structs and can be accessed without keeping the
-source file open.
+source file open. You can also use this library to construct and serialize an XML file.
 
 Used in https://github.com/speedata/goxpath
 
@@ -57,6 +57,45 @@ func main() {
 }
 ```
 
+Constructing an XML file:
+
+~~~go
+package main
+
+import (
+	"encoding/xml"
+	"fmt"
+
+	"github.com/speedata/goxml"
+)
+
+func main() {
+	d := goxml.XMLDocument{}
+	root := &goxml.Element{Name: "root"}
+	d.Append(root)
+	cd := goxml.CharData{Contents: "\n   "}
+	root.Append(cd)
+	elt1 := &goxml.Element{Name: "element"}
+	elt1.SetAttribute(xml.Attr{Name: xml.Name{Local: "attr"}, Value: "element 1"})
+	elt1.SetAttribute(xml.Attr{Name: xml.Name{Local: "attr2"}, Value: "some <value> &'"})
+	root.Append(elt1)
+	root.Append(cd)
+	elt2 := &goxml.Element{Name: "element"}
+	elt2.SetAttribute(xml.Attr{Name: xml.Name{Local: "attr"}, Value: "element 2"})
+	root.Append(elt2)
+	root.Append(goxml.CharData{Contents: "\n"})
+	fmt.Println(d.ToXML())
+}
+~~~
+
+prints
+
+```xml
+<root>
+   <element attr="element 1" attr2="some &lt;value> &amp;'" />
+   <element attr="element 2" />
+</root>
+```
 
 
 License: BSD-3-Clause License

xmldecoder.go

@@ -170,6 +170,19 @@ func (elt Element) Children() []XMLNode {
 	return elt.children
 }
 
+// SetAttribute appends attr to the list of attributes of elt. If an attribute
+// of this name already exists, the existing one will be discarded.
+func (elt *Element) SetAttribute(attr xml.Attr) {
+	var newAttributes = make([]xml.Attr, 0, len(elt.attributes)+1)
+	for _, curattr := range elt.attributes {
+		if curattr.Name != attr.Name {
+			newAttributes = append(newAttributes, curattr)
+		}
+	}
+	newAttributes = append(newAttributes, attr)
+	elt.attributes = newAttributes
+}
+
 // Attributes returns all attributes for this element
 func (elt Element) Attributes() []*Attribute {
 	var attribs []*Attribute