Fix `thread_mattr_accessor` share variable superclass with subclass

The current implementation of `thread_mattr_accessor` set variable
sharing superclass with subclass. So the method doesn't work as documented.

Precondition

    class Account
      thread_mattr_accessor :user
    end

    class Customer < Account
    end

    Account.user = "DHH"
    Account.user  #=> "DHH"
    Customer.user = "Rafael"
    Customer.user # => "Rafael"

Documented behavior

    Account.user  # => "DHH"

Actual behavior

    Account.user  # => "Rafael"

Current implementation set variable statically likes `Thread[:attr_Account_user]`,
and customer also use it.

Make variable name dynamic to use own thread-local variable.

0 files changed, 0 insertions, 0 deletions