feat: add String method to Document

this is really helpful for tests and debugging

document.go

@@ -13,6 +13,7 @@ type Document struct {
 	node *html.Node
 }
 
+func (d *Document) String() string                  { return outerHTML(d.node) }
 func (d *Document) NodeType() spec.NodeType         { return nodeType(d.node.Type) }
 func (d *Document) CloneNode(deep bool) spec.Node   { return NewNode(cloneNode(d.node, deep)) }
 func (d *Document) IsSameNode(other spec.Node) bool { return isSameNode(d.node, other) }

element.go

@@ -165,14 +165,7 @@ func (e *Element) SetOuterHTML(s string) {
 	e.node.Parent.RemoveChild(e.node)
 }
 
-func (e *Element) OuterHTML() string {
-	var buf bytes.Buffer
-	err := html.Render(&buf, e.node)
-	if err != nil {
-		panic(err)
-	}
-	return buf.String()
-}
+func (e *Element) OuterHTML() string { return outerHTML(e.node) }
 
 type siblingElements struct {
 	firstChild *html.Node

node.go

@@ -11,6 +11,15 @@ import (
 	"github.com/crhntr/dom/spec"
 )
 
+func outerHTML(node *html.Node) string {
+	var buf bytes.Buffer
+	err := html.Render(&buf, node)
+	if err != nil {
+		return ""
+	}
+	return buf.String()
+}
+
 func nodeType(nodeType html.NodeType) spec.NodeType {
 	switch nodeType {
 	case html.TextNode: