Backport 1536

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This backports #1536 to the 2-8-stable-branch.

Why

Though mail v2.8 enabled turning TLS/STARTTLS options off (this surfaced as net-smtp's default value for starttls went from false to :auto), it also introduced some new problems:

• having ssl false would ignore any starttls-setting
• enable_starttls: false ignores the enable_starttls_auto-flag.

These issues were fixed on mainline but I think it's worth to have these in a new 2.8 release as well.

Impact

Possible outcomes:

• having either ssl or tls truthy ánd having truthy values for any of enable_starttls*-flags set will now raise an ArgumentError.
  While mail is stricter in this regard, net-smtp would already consider this a non-acceptable config.
• another scenario: a config with enable_starttls false and enable_starttls_auto true resulted in starttls :auto <v2.8, in v2.8 this resulted in starttls false and would now again result in:auto.

Did I miss anything?

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@ahorek @sebbASF review appreciated.

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Merging as this was reviewed/discussed in the master-PR.