Map to_s rather than to_s array

[hubert]

As written currently, we are calling to_s on an array of data's keys then checking whether the @_her_root_element is part of the string. I'm betting that the intention is to call to_s on the individual key and then check whether the root element is included in the stringified array of keys.

There are some incidental side effects

• if the root element is nil, root_element_included always returns true.
• if the root element is 'foo' and there is an key in the data hash 'foobar', root_element_included? will return true.