ArtifactResolve: change order of elements to satisfy ADFS

fixes #535

Author: crewjam

schema.go

@@ -353,12 +353,15 @@ func (r *ArtifactResolve) Element() *etree.Element {
 	if r.Issuer != nil {
 		el.AddChild(r.Issuer.Element())
 	}
-	artifact := etree.NewElement("samlp:Artifact")
-	artifact.SetText(r.Artifact)
-	el.AddChild(artifact)
 	if r.Signature != nil {
+		// ADFS requires that <Signature> come before <Artifact>.
+		// ref: https://github.com/crewjam/saml/issues/535
+		// ref: https://www.wiktorzychla.com/2017/09/adfs-and-saml2-artifact-binding-woes.html
 		el.AddChild(r.Signature)
 	}
+	artifact := etree.NewElement("samlp:Artifact")
+	artifact.SetText(r.Artifact)
+	el.AddChild(artifact)
 	return el
 }