Rename Result.Headers to Result.Header.

This way, it's consistent with http.Header in the response.
diff --git a/openstack/identity/v3/tokens/results.go b/openstack/identity/v3/tokens/results.go
index 0b6a55b..c31c52f 100644
--- a/openstack/identity/v3/tokens/results.go
+++ b/openstack/identity/v3/tokens/results.go
@@ -27,7 +27,7 @@
 	var token Token
 
 	// Parse the token itself from the stored headers.
-	token.ID = r.Headers.Get("X-Subject-Token")
+	token.ID = r.Header.Get("X-Subject-Token")
 
 	err := mapstructure.Decode(r.Body, &response)
 	if err != nil {