include hash source for hashrocket hash attributes

Fix an issue where, if a tag has hash attributes consisting only of of strings and symbols and is written in hashrocket style, those attributes were not present in the extracted script source passed to rubocop, and so were going unchecked.
Why are we calling `merge` here? If I remove it and adjust the corresponding test to not check for `.merge(Hash.new(0))` in the output, won't this be the same? Or is there some Rubocop check I'm not aware of?