Rename Result.Headers to Result.Header.

This way, it's consistent with http.Header in the response.
diff --git a/openstack/objectstorage/v1/accounts/results.go b/openstack/objectstorage/v1/accounts/results.go
index 964a604..e42577a 100644
--- a/openstack/objectstorage/v1/accounts/results.go
+++ b/openstack/objectstorage/v1/accounts/results.go
@@ -20,7 +20,7 @@
 	}
 
 	metadata := make(map[string]string)
-	for k, v := range gr.Headers {
+	for k, v := range gr.Header {
 		if strings.HasPrefix(k, "X-Account-Meta-") {
 			key := strings.TrimPrefix(k, "X-Account-Meta-")
 			metadata[key] = v[0]
@@ -37,5 +37,5 @@
 // Extract returns the unmodified HTTP headers and any error conditions encountered during the
 // metadata update.
 func (ur UpdateResult) Extract() (http.Header, error) {
-	return ur.Headers, ur.Err
+	return ur.Header, ur.Err
 }