Added support for multipart messages

[koke24]

Thank you for this software!

I was using dumbster to test multipart messages and I found it didn't handle them right, missing some newline characters. I have written a small change to handle them and maybe you want to insert it in your code.

Best regards

[gerardnico]

Man I got the same problem. Can we merge this commit ?

[nfalco79]

@kirviq could you provide a quick check and if ok merge it into master and perform a release? Otherwise every test will return a wrong numbers of sent emails @koke24 could you provide some test cases?