Add missing test case for writing unknown attributes
HashWithIndifferentAccess#dup doc [ci skip]

The phrase "exact copy" in the existing docmentation is somewhat
misleading.
Merge pull request #15440 from ilikepi/pr_docs_activesupport_hwia_dup

HashWithIndifferentAccess#dup doc [ci skip]
Merge pull request #15437 from sgrif/sg-missing-test-case

Add missing test case for writing unknown attributes
Return a null column when no column exists for an attribute