Allow custom headers when sending email

[marcari]

The current implementation forces the reply-to and does not allow to pass custom headers. This pull request makes it possible to change the default behavior and adds a header parameter.

[Natim]

Hello @marcari, thanks a lot, could you please add two tests for that?

• One to make sure if we don't provide headers we've got the Reply-To header.
• Another one to make sure if we provide headers they all are present in the mail header.

[marcari]

The two tests have been added and pass on my local dev box.

[Natim]

Thanks a lot.

And then I realized I don't have the contributor bit anymore :(

[magopian]

I don't either :/

[zebuline]

Thanks for the PR