`connection.type_to_sql` returns a `String` for unmapped types.

Closes #13146.

This fixes an error when using:

```
change_colum :table, :column, :bigint, array: true
```

activerecord/CHANGELOG.md

+*   `type_to_sql` returns a `String` for unmapped columns. This fixes an error
+    when using unmapped array types in PG
+
+    Example:
+
+        change_colum :table, :column, :bigint, array: true
+
+    Fixes #13146.
+
+    *Jens Fahnenbruck*, *Yves Senn*
+
 *   Fix `QueryCache` to work with nested blocks, so that it will only clear the existing cache
     after leaving the outer block instead of clearing it right after the inner block is finished.
 

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -690,7 +690,7 @@ def type_to_sql(type, limit = nil, precision = nil, scale = nil) #:nodoc:
 
           column_type_sql
         else
-          type
+          type.to_s
         end
       end
 

activerecord/test/cases/adapter_test.rb

@@ -178,6 +178,10 @@ def test_select_all_always_return_activerecord_result
       result = @connection.select_all "SELECT * FROM posts"
       assert result.is_a?(ActiveRecord::Result)
     end
+
+    test "type_to_sql returns a String for unmapped types" do
+      assert_equal "special_db_type", @connection.type_to_sql(:special_db_type)
+    end
   end
 
   class AdapterTestWithoutTransaction < ActiveRecord::TestCase