WIP: Fix the AS::Callbacks terminator regression from 4.2.3

Document Bearer prefix for Authorization header [ci skip]

Removed Mocha from app generators tests

skipped assertion on datetime seconds precision as it is only valid for newer mysql verions        

It doesn't make sense to access the response object before a request is
made (how was a response object created without making a request?)  This
commit splits testing default headers and default header mutation tests
and removes access to the pre-request response object.

these should really be multiple tests.

When the response object is `to_a`'d, that means it's been written to
the socket.  It doesn't make sense to mutate the response object after
it's been written (and this may raise an exception in the future).

When I originally reviewed the #20317, I believe these changes were
present, but it appears that it was later updated so that they were
removed. Since Travis hadn't re-run the build, this slipped through.

And we are passing them as separate types in the query, which means 0
precision is still not supported by older versions of MySQL. I also
missed a handful of other cases where they need to be conditionally
applied.

Specifically, versions of MySQL prior to 5.6 do not support this, which
is what's used on Travis by default. The method `mysql_56?` appeared to
only ever be used to conditionally apply subsecond precision, so I've
generalized it and used it more liberally.

This should fix the test failures caused by #20317

AR: take precision into count when assigning a value to timestamp
attribute

Timestamp column can have less precision than ruby timestamp
In result in how big a fraction of a second can be stored in the
database.

  m = Model.create!
  m.created_at.usec == m.reload.created_at.usec
    # => false
    # due to different seconds precision in Time.now and database column

If the precision is low enough, (mysql default is 0, so it is always low
enough by default) the value changes when model is reloaded from the
database. This patch fixes that issue ensuring that any timestamp
assigned as an attribute is converted to column precision under the
attribute.

Updated Mime Negotiations docs [ci skip]

As we all know that Accessing mime types via constants is deprecated. Now, we are using `Mime::Type[:JSON]` instead of `Mime::JSON`

remove warning from Calculations#sum

This removes the following warning.
```
activerecord/lib/active_record/relation/calculations.rb:74: warning: `&' interpreted as argument prefix
```

stop using deprecated method in mailers controller

Fix arguments of `AR::Calculations#sum`

Remove leading slash from path

Remove not used argument `table_name` of `sanitize_sql_for_conditions`

The leading slash denotes an absolute path,
rather than a relative one (which is more popular)

[ci skip]

This argument was needen when `sanitize_sql_for_conditions` internally
called `sanitize_sql_hash_for_conditions`.
But `sanitize_sql_hash_for_conditions` was deprecated
(https://github.com/rails/rails/commit/eb921000a11bc87a3b001164fc367b84aee584c5)
and deleted
(https://github.com/rails/rails/commit/3a59dd212315ebb9bae8338b98af259ac00bbef3)
(https://github.com/rails/rails/commit/4bd089f1d93fa168b0ae17dd8c92a5157a2537d7).

Rails 4.2.3 AS::Callbacks will not halt chain if `false` is returned.
That is the behavior of specific callbacks like AR::Callbacks and
AM::Callbacks.

Add tests for sanitize named bind arity

Accessing mime types via constants is deprecated.

Arguments of `#sum` does not match with other shortcuts methods
(count, average, minimum, and maximum).
This commit fix these two points:

* call `super` with only block arguments
  First argument of `super` method, `Enumerable#sum`, is `identity`
  and first argument of `AR::Calculations#sum` is `column_name`.
  `Enumerable#sum` does not expect `column_name` to be passed.
* Change first argument of `sum` from array arguemnt to single
  argument to match other shortcuts methods. When `sum` accept
  array arguemnt, user can pass multi arguments and an exception is
  raised from `calculate`.

The flash middleware shouldn't know how to look up the session object.
Just ask the request for that information.

We can know whether or not there is a content type object, and just exit
early.  There is no need to `try` so hard.

Added assertion for error messages for nil and unknown delivery methods

introduce `conn.data_source_exists?` and `conn.data_sources`.

These new methods are used from the Active Record model layer to
determine which relations are viable to back a model. These new methods
allow us to change `conn.tables` in the future to only return tables and
no views. Same for `conn.table_exists?`.

The goal is to provide the following introspection methods on the
connection:

* `tables`
* `table_exists?`
* `views`
* `view_exists?`
* `data_sources` (views + tables)
* `data_source_exists?` (views + tables)

Update i18n.md [ci skip]