Simplify code by taking advantage of latest mocha (v0.13.0).

`#as_json` isolates options when encoding a hash. Closes #8182

voloko/sdoc seems to be maintained again, in particular
the issues that made us fork it are resolved.

Merge and add tests related to #5215

Setting options in a custom `#as_json` method had side effects.
Modifications of the `options` hash leaked outside and influenced
the conversion of other objects contained in the hash.

This only works with mocha v0.13.0 or later.

Note that this also fixes a few subtle bugs present in the current
implementation :-

* Mocha was raising a `MiniTest::Assertion` instead of a
  `Mocha::ExpectationError` as intended. The latter is not recognized by
  MiniTest as an assertion failure and so it is recorded as a test
  *error*, not a test *failure* as it ought to. This leads to
  potentially confusing output in the test results.

* Mocha verification should happen as part of the test. The verification
  of expectations is equivalent to a set of assertions. These assertions
  should happen as *part of* the test so that they have a chance to
  cause the test to fail, and not just as part of the teardown. Also if
  an assertion fails during the test, then there is no need to verify
  expectations, because only the first assertion failure is normally
  reported and all subsequent bets are off.

* Expectation verification should be counted as an assertion. Mocha
  cannot record each expectation verification as an assertion, because
  we weren't passing in an assertion counter to `#mocha_verify`.

Thanks @toretore.
https://github.com/rails/rails/commit/9e4c41c903e8e58721f2c41776a8c60ddba7a0a9#commitcomment-2134706

Match classes without to_s conversions.

There is no need in `value_under_case.class.to_s` conversion since case already matches classes with `Class === value_under_case`.

Test Hash#diff deprecation, also prevent deprecation message in AS tests

Removing warning : ambiguous first argument

Two threads may be in method_missing at the same time. If so, they might
both try to define the same delegator method.

Such a situation probably wouldn't result in a particularly spectacular
bug as one method would probably just be overridden by an identical
method, but it could cause warnings to pop up. (It could be worse if
method definition is non-atomic in a particular implementation.)

(We will also need this mutex shortly anyway, see #8127.)

Removing warning : assigned but unused variable

Remove unwanted transaction when has one association is built

Keep the code related to serialization in Serialization module.

We should not need any `serialized_attributes` checks outside `ActiveRecord::AttributeMethods::Serialization` module.

Trivial documentation fix for ActiveModel::Naming comment [ci skip]

get is the most common usage, and match without an explicit verb
was disallowed in 56cdc81c.

[ci skip]

Most apps upgrading from 3.x will have options for mass assigment in
their application.rb and environments/*.rb config files. Rather than
just raising a NoMethodError when copying the config, this commit
adds a warning message until either the protected_attributes gem
is installed or the relevant config options are removed.

Deprecate Hash#diff.

It's no longer used in Rails any more.

See https://github.com/rails/rails/pull/8142\#issuecomment-10227297 for more

These were removed with ActiveRecord::Model in
https://github.com/rails/rails/commit/9e4c41c903e8e58721f2c41776a8c60ddba7a0a9#L15L156

Rather than just changing it and hoping for the best.

Requested by @jeremy:

https://github.com/rails/rails/commit/ba1544d71628abff2777c9c514142d7e9a159111#commitcomment-2106059

Removing duplication in callback normalization.

These two things were 100% identical.

:counter_cache option for  to support custom named counter caches

So that the scope may be a NullRelation and return a result without
executing a query.

Fixes #7928

Fixes #8102.

I couldn't find a nicer way to deal with this than delegate the call to
 #scope, which will be a NullRelation when we want it to be.