remove unwanted `to_sym` call.
Fix redefine a has_and_belongs_to_many inside inherited class

After ad7b5efb55bcc2e0ccd3e7f22a81e984df8676d1, which changed how
has_an_belongs_to_many used to work, we start raising an error when
redefining the same has_an_belongs_to_many association. This commits fix
that regression.

[Fixes #14983]
Deprecate decimal columns being automatically treated as integers

With ActiveRecord::Properties, we now have a reasonable path for users
to continue to keep this behavior if they want it. This is an edge case
that has added a lot of complexity to the code base.
Merge pull request #15359 from kuldeepaggarwal/f-remove-unwanted_to_sym

remove unwanted `to_sym` call.
Merge pull request #15358 from arthurnn/redefine_habtm_fix

Fix redefine a has_and_belongs_to_many inside inherited class
Merge pull request #15295 from sgrif/sg-deprecate-decimals

Deprecate decimal columns being automatically treated as integers