Rename Result.Headers to Result.Header.

This way, it's consistent with http.Header in the response.
diff --git a/openstack/objectstorage/v1/accounts/requests.go b/openstack/objectstorage/v1/accounts/requests.go
index 2706197..a364bcc 100644
--- a/openstack/objectstorage/v1/accounts/requests.go
+++ b/openstack/objectstorage/v1/accounts/requests.go
@@ -46,7 +46,7 @@
 		MoreHeaders: h,
 		OkCodes:     []int{204},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }
@@ -100,7 +100,7 @@
 		MoreHeaders: h,
 		OkCodes:     []int{204},
 	})
-	res.Headers = resp.HttpResponse.Header
+	res.Header = resp.HttpResponse.Header
 	res.Err = err
 	return res
 }