option_tags coerced to "" instead of nil

Closes #7404

Refactor AR::Result or inherits. Because we have redundant codes about column_types.

Change mysql tests for strict mode disabled to work with new sql_mode default in MySQL 5.6.6-m9.

Since this MySQL version, the sql_mode default value is `NO_ENGINE_SUBSTITUTION`.

Reason since MySQL 5.6.6-m9 the `sql_mode` default value is
`NO_ENGINE_SUBSTITUTION`.
This default parameter change is out of control from Rails.

This test verifies Rails not overriding the default `@@GLOBAL.sql_mode` value
by checking if `@@GLOBAL.sql_mode` is the same as `@@SESSION.sql_mode`.

Fixed some documentation mistakes

Use instance_accessor: false instead of instance_writer.

Fix #6975. Round usec when writing timestamp attribute.

* Use each_key instead of generating intermediate keys array.
* Use each_with_object instead of inject to build hash.
* Use ternary to return instead of if + assignment.

This argument is there because that's the signature of Kernel#load.

This reverts commit bf3fa34e.

Fix punctuation in config templates [ci skip]

Access @rs only through attr_accessor

Ruby does not pass the nesting to const_missing (unfortunately).
That second argument was there in case that changed, Yehuda
sent a patch to MRI

    http://bugs.ruby-lang.org/issues/2740

but there is not much movement there and Matz told me in
Amsterdam there was no immediate plan to pass the nesting.
So let's go back to implement what happens now, and if
in the future we get the nesting then we will adapt this.

Double-checked this with Mr Katz.

This method was added to be shared between update_attribute and
update_column in 50725cec, but since
update_attribute was removed, and update_column has changed to delegate
to update_columns, the method is not used anywhere anymore.

Also remove "key.to_s" conversion when raising readonly error, since
the key is being interpolated.

Fix "last equality wins" logic in relation merge

This is a real fix (as compared to the band-aid in b127d86c), which uses
the recently-added equality methods for ARel nodes. It has the side
benefit of simplifying the merge code a bit.

Use inversed parent for first and last child of has_many association

Added X-Content-Type-Options to the header defaults.

With a value of "nosniff", this prevents Internet Explorer from MIME-sniffing a response away from the declared content-type.

Refactor multiparameter attributes assignment implementation.