fix: Mail.add_header doesn't accept a dict

[hassaku63]

Fixes #997

Headers are correctly added even if the argument of add_header is of type dict.

The current spec seems to intend that only the first key/value is added when a dict type argument is given. I couldn't figure out why this implementation.
I think the spec should apply to all keys/values that a dict argument has. The current spec is non-intuitive and confusing to users.

However, changes for supports to multiple key/value pairs could result in "breaking change", so the policy in this pull request was to keep changes to a minimum.

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