Add function to get all header keys from Email object

[matthiasbeyer]

This is helpful if the user of the Email object does not know which headers exist.

[afrantzis]

Thanks for the patch! Adding such a function makes sense. I suggest we go with the following name (to match RFC terminology) and signature (to hide implementation details):

pub fn header_field_names(&self) -> Vec<&str>

Also, could you please add a doc comment and test (see tests/test_fields.rs) for this?

[matthiasbeyer]

If you're fine with the changes I'll fixup-rebase 😄

[afrantzis]

Thanks for the update. Besides a small test improvement (see inline comment), looks good.

[afrantzis]

We also need to check that what's returned by header_field_names is exhaustive (i.e., all field names present in the email are included).

[matthiasbeyer]

Doesn't the assertion do this? I mean, if one header_field_name is NOne when asking for header_field(header_field_name), that assert fails, right?

[afrantzis]

The current assertion correctly checks that all names returned by header_field_names() are proper/present field names, but not that these names are the full list of names present in the email. For example, if the header contains "To", "Cc", "From" fields, but due to some bug header_field_names() returns only "To" and "Cc", this test will still pass.