Test `except!` in other cases too
Relax mail gem constraint from ~> 2.5.4 to ~> 2.5, >= 2.5.4

This allows Rails users to install mail 2.6 which relaxes
the mime-types dependency, which is a big win for a lot of people.

Previously, the mail gem restricted mime-types to ~> 1.16
but now it has expanded to [">= 1.16", "< 3"]

And the mime-types maintainer will also be checking that
2.x releases don't break mail.

See https://github.com/mikel/mail/pull/713
https://rubygems.org/gems/mail/versions/2.6.0
Revert "test pg, we don't care about the internal state of `column#default`."

Revert "test pg, remove unused column assignments. Follow up to 254cdf47"

Related to #15492

This reverts commit 254cdf4728291277f3fbaa854f34495030e476b4.
This reverts commit 4bcf9029452e0c760af04faab6b549710401e8cf.

There are public methods that assume `Column#default` is type casted.
The return value of `Column#default` is publicly relevant and should not change.

/cc @sgrif
Routes specifying 'to:' must be a string that contains a "#" or a rack
application.  Use of a symbol should be replaced with `action: symbol`.
Use of a string without a "#" should be replaced with `controller: string`.
oops! :bomb:
Merge pull request #15447 from akshay-vishnoi/test-cases

Test `except!` in other cases too
force table creation
Merge pull request #15493 from bf4/relax_mail_dep

Relax mail gem constraint from ~> 2.5.4 to ~> 2.5