Rename Result.Headers to Result.Header.

This way, it's consistent with http.Header in the response.
diff --git a/openstack/identity/v3/tokens/requests.go b/openstack/identity/v3/tokens/requests.go
index 3a21658..dfef0ce 100644
--- a/openstack/identity/v3/tokens/requests.go
+++ b/openstack/identity/v3/tokens/requests.go
@@ -242,7 +242,7 @@
 	if result.Err != nil {
 		return result
 	}
-	result.Headers = response.HttpResponse.Header
+	result.Header = response.HttpResponse.Header
 	return result
 }
 
@@ -258,7 +258,7 @@
 	if result.Err != nil {
 		return result
 	}
-	result.Headers = response.HttpResponse.Header
+	result.Header = response.HttpResponse.Header
 	return result
 }