wrong table name. bug introduced in my last PR

[burnt43]

My code changes the table_name class variable of an ActiveRecord::Base, auto_upgrade! was likely to fail if you have a has_and_belongs_to_many. This is because I was using gsub! on table_name itself and not a clone of the table names in my calculating the join table name.

Basically the situation is like the following:

Before:
SchoolTeacher.table_name is 'school_teachers'
SchoolStudent.table_name is 'school_students'.

The code I added would make the join table 'school_students_teachers' like Rails 4 expects. However, afterwards there is an unintended consequence.

After:
SchoolTeacher.table_name is 'teachers' NOT GOOD!!!
SchoolStudent.table_name is 'school_students'.

See PR #54
See commit fc4ccf5

I have a fix for this!

[burnt43]

I fixed this in #56