fetch.Error has unique headers

[glossd]

Headers according to HTTP 1.1 don't have to be unique and can be repeated.
fetch.Error.Headers filters them out and takes the last one.
fetch.Response handles it with DuplicateHeaders field and Headers() method.
fetch.Error should handle it in the same, but it would break backward compatibility.

[glossd]

Actually, I don't like Headers() method. It's nicer if it's a field. If I'm breaking backward compatibility then, I should go for the nice one everywhere.