Change all `MiniTest` to `Minitest` since, `MiniTest` namespace has been renamed to `Minitest`
Ref: https://github.com/seattlerb/minitest/blob/master/History.txt
Fix nil assignment to polymorphic belongs_to

Assigning nil to a polymorphic belongs_to would nullify its _id field by
not its _type field.

Fixes failing test from c141dfc838a5dca9f197814410fa5d44c143129c.
Regression from 1678e959e973de32287b65c52ebc6cce87148951.
document how to access mail previews. [ci skip]

/cc @pixeltrix
Always point to latest rbx and jruby in travis
make clear that config/secrets.yml passes through ERB and therefore supports ENV
Fix to_param when attribute has multibyte character
Merge pull request #13386 from rono23/fix-to-param

Fix to_param when attribute has multibyte character
Unused classes in test case
Merge pull request #13366 from vipulnsward/minitest-namespace-change

Change all `MiniTest` to `Minitest`
set encoding UTF-8 for accepting multibyte character
Fix typo: data --> date
Merge pull request #13399 from ghiculescu/patch-1

Fix typo: data --> date [ci-skip]
Merge pull request #13398 from kuldeepaggarwal/build-fixes

set encoding UTF-8 for accepting multibyte character
fix typos and grammar mistake [ci skip]
Merge pull request #13400 from nishant-cyro/docs

fix typos and grammar mistake [ci skip]
Unused classes in AS tests
Prevent creation of instance methods when `instance_reader = false`, Grammar checks, Conditional statements combined