set the configured #inheritance_column on #become (#7503)

I had to create a new table because I needed an STI table,
which does not have both a "type" and a "custom_type"

the test fails with:
  1) Error:
test_alt_becomes_works_with_sti(InheritanceTest):
NoMethodError: undefined method `type=' for #<Cabbage id: 1, name: "my cucumber", custom_type: "Cucumber">
    /Users/username/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:432:in `method_missing'
    /Users/username/Projects/rails/activerecord/lib/active_record/attribute_methods.rb:100:in `method_missing'
    /Users/username/Projects/rails/activerecord/lib/active_record/persistence.rb:165:in `becomes'
    test/cases/inheritance_test.rb:134:in `test_becomes_works_with_sti'
    test/cases/inheritance_test.rb:140:in `test_alt_becomes_works_with_sti'

1 files changed, 14 insertions, 0 deletions

diff --git a/activerecord/test/models/vegetables.rb b/activerecord/test/models/vegetables.rb
new file mode 100644
index 0000000000..59cedfd9f5
--- /dev/null
+++ b/activerecord/test/models/vegetables.rb
@@ -0,0 +1,14 @@
+class Vegetable < ActiveRecord::Base
+
+  validates_presence_of :name
+
+  def self.inheritance_column
+    'custom_type'
+  end
+end
+
+class Cucumber < Vegetable
+end
+
+class Cabbage < Vegetable
+end