honoring retry-after response header?

[bulletblp4]

Looking at the code it seems like the public method call Push / PushWithContext doesn't return response headers such as retry-after also seems like internally (within the lib) we aren't handling the retry cases either (for transient or any other errors).

Are we completely overlooking retry-after sent by apple in this lib or is it that apple never sends that header and it not useful or is there any other way I can access that header currently seems like I can only access these values https://github.com/sideshow/apns2/blob/master/response.go#L111-L132.

using github.com/sideshow/apns2 v0.20.0

1. What version of Go are you using? go version
   go version go1.13.1 darwin/amd64

2. What OS and processor architecture are you using? go env - Seems not related to the question hence skipping this for now