tls: Reorder some logic to avoid subtle, undocumented behavior
tls: Reorder some logic to avoid subtle, undocumented behavior

By calling SetTLSAddress, the acme package reset the challenge provider
to the default one instead of keeping the custom one we specified before
with SetChallengeProvider. Yikes. This means that Caddy would try to
open a listener on port 443 even though we should have been handling it
with our provider, causing the challenge to fail, since usually port 443
is in use.

So this change just reorders the calls so that our provider takes
precedence.

cf. https://github.com/xenolf/lego/pull/292

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