Fix pluck when columns/tables are reserved words.

Backport #7536 to fix #8968.

Conflicts:

	activerecord/CHANGELOG.md
	activerecord/lib/active_record/relation/calculations.rb
Add NumberHelper missing dependency

symbolize_keys depends on hash/keys AS core extension
Merge pull request #9113 from rosenfeld/patch-1

Add NumberHelper missing dependency
Add another NumberHelper missing dependency

Another missing dependency, now affecting #number_to_percentage.

It depends on reverse_merge.
Merge pull request #9114 from rosenfeld/patch-2

Add another NumberHelper missing dependency
Merge pull request #9043 from senny/backport_7536

Backport: Fix pluck when columns/tables are reserved words.
Fix markdown syntax in actionpack CHANGELOG.

Starting a line with an octothorpe makes an <h1>
Merge pull request #9078 from senny/6865_ar_count_with_uniq

`#count` in conjunction with `#uniq` performs distinct count.
Conflicts:
	activerecord/CHANGELOG.md
Duplicate column_defaults properly

Backport c517602 to fix #6115

Deleted:
    activerecord/lib/active_record/core.rb

Conflicts:
    activerecord/test/cases/base_test.rb
This pull requests addresses ORA-01400 errors and also supports #6115 issue tested.

Issue #6115 has been fixed and tested with
the attribute `:null => false, :default => ""`
However `:null => false` attribute is not necessary to test this issue,
which causes many ORA-01400 errors with Oracle enhanced adapter.
Merge pull request #9159 from lexmag/col_defaults-dup-fix

Backport: Duplicate column_defaults properly