Fix unable to annotate error when table_name is frozen

[ryu39]

I found that annotate command is failed when table_name of model class is frozen.
Please see the sample model class code.

# frozen_string_literal: true

class User < ApplicationRecord
  self.table_name = 'tbl_users'
end

When annotate command is executed to the model file, it is failed with following message .
(Messages with --trace option are backtraces.txt.)

Unable to annotate app/models/user.rb: can't modify frozen String
Unable to annotate app/models/user.rb: no implicit conversion of nil into Array

This is seemed to be caused by this line.
table_name.to_s.slice!(klass.table_name_prefix) changes table_name string itself, but table_name is frozen(by frozen_string_literal magic comment), so can't modify frozen String error is raised.

I fixed this error. Please check this PR.

[ctran]

Haha, that was some lousy coding!

[qsona]

@ctran hi, I had trouble with same problem (on Rails 5.2) and found it's already fixed with this PR. Could you please release it sooner? Thank you for your great work.