Fix frozen string concatenation by indicating that it's mutable

References 89bcca5

activerecord/lib/active_record/errors.rb

@@ -120,13 +120,13 @@ module ActiveRecord
     def initialize(adapter = nil, message: nil, table: nil, foreign_key: nil, target_table: nil, primary_key: nil)
       @adapter = adapter
       if table
-        msg = <<~EOM
+        msg = +<<~EOM
           Column `#{foreign_key}` on table `#{table}` has a type of `#{column_type(table, foreign_key)}`.
           This does not match column `#{primary_key}` on `#{target_table}`, which has type `#{column_type(target_table, primary_key)}`.
           To resolve this issue, change the type of the `#{foreign_key}` column on `#{table}` to be :integer. (For example `t.integer #{foreign_key}`).
         EOM
       else
-        msg = <<~EOM
+        msg = +<<~EOM
           There is a mismatch between the foreign key and primary key column types.
           Verify that the foreign key column type and the primary key of the associated table match types.
         EOM