Make SP check more certs in IDP metadata (#353)

From
https://www.oasis-open.org/committees/download.php/56785/sstc-saml-metadata-errata-2.0-wd-05.pdf
```
[E62]A use value of "signing" means that the contained key information is applicable to both signing
and TLS/SSL operations performed by the entity when acting in the enclosing role.

A use value of "encryption" means that the contained key information is suitable for use in wrapping
encryption keys for use by the entity when acting in the enclosing role.

If the use attribute is omitted, then the contained key information is applicable to both of the above uses.
```

We need to include certificates both when they have a "use" attribute of
"signing" as well as when the "use" attribute is missing.

Fixes #352

SAML input from @simmel.

service_provider.go

@@ -288,22 +288,15 @@ func (sp *ServiceProvider) GetSLOBindingLocation(binding string) string {
 // signed by the IDP in PEM format, or nil if no such certificate is found.
 func (sp *ServiceProvider) getIDPSigningCerts() ([]*x509.Certificate, error) {
 	var certStrs []string
+
+	// We need to include non-empty certs where the "use" attribute is
+	// either set to "signing" or is missing
 	for _, idpSSODescriptor := range sp.IDPMetadata.IDPSSODescriptors {
 		for _, keyDescriptor := range idpSSODescriptor.KeyDescriptors {
-			if keyDescriptor.Use == "signing" {
-				certStrs = append(certStrs, keyDescriptor.KeyInfo.Certificate)
-			}
-		}
-	}
-
-	// If there are no explicitly signing certs, just return the first
-	// non-empty cert we find.
-	if len(certStrs) == 0 {
-		for _, idpSSODescriptor := range sp.IDPMetadata.IDPSSODescriptors {
-			for _, keyDescriptor := range idpSSODescriptor.KeyDescriptors {
-				if keyDescriptor.Use == "" && keyDescriptor.KeyInfo.Certificate != "" {
+			if keyDescriptor.KeyInfo.Certificate != "" {
+				switch keyDescriptor.Use {
+				case "", "signing":
 					certStrs = append(certStrs, keyDescriptor.KeyInfo.Certificate)
-					break
 				}
 			}
 		}