The latter is just the first name in the 'server_name' directive.
The former uses the HTTP Host header, if provided.
This fixes the redirect
from http://zephyr.humbughq.com
to https://zephyr.humbughq.com
(imported from commit be47b05f4f055bb2d1d82aebbe155579f49c538d)
This will cause SSL errors for anyone still using the deprecated
app.humbughq.com name, which we concluded is (almost?) nobody.
(imported from commit 7f3c149a4064e7bdae8ec944f2bb8a482df6f90d)