`Tags::Base#select_content_tag`.
Previously, passing a falsey value to `include_blank` would be ignored if the
field was required, and a blank line would still be inserted. The following will
now raise instead of quietly failing:
`select("post", "category", %w(a required field), { include_blank: false }, required: 'required')`
AS does the following inside Time.find_zone! ...
`ActiveSupport::TimeZone[time_zone] || TZInfo::Timezone.get(time_zone)`
and given that the test is stubbing AS::TZ[] we don't need the removed
code.