SparkPostMessage wrapper for Django EmailMessage ignores extra_headers field

[wooyek]

I'm trying to set a HTTP version of List-Unsubscribe header into a message and it gets dropped during conversion from EmailMessage.extra_headers to sparpost transmissions.send params, where it should probably land as custom_headers (Used to set any headers associated with transmission).

Anyway even patching SparkPostMessage class and making sure custom_headers get posted as model['content']['headers'] has no effect, HTTP variant of List-Unsubscribe header is missing it the message received.

Custom List-Unsubscribe is present in the message received only when transactional option is set, but then the message lacks a MAILTO variant.

[wooyek]

Solved by #160