Fix microdata itemrefs

Currently our microdata parser fails to recognize itemrefs, as shown by the following failed tests from the online microdata test suite:

Test 0062: @itemref to single id
Test 0063: @itemref generates property values
Test 0064: @itemref to single id with different types
Test 0065: @itemref to multiple ids
Test 0066: @itemref with chaining
Test 0067: Shared @itemref

It appears this ability was previously working as of version 2.1, but the following code-change caused this bug and introduced a regression into version 2.2:

https://github.com/apache/any23/commit/60e93a76748e53c413529409fb545e2245013639#r31047991