Change consolidated fields code for docutils 0.22

[tristanlatr]

From the release notes:

Document Tree / Docutils DTD
Allow multiple elements in a <definition_list_item> (third-party writers may need adaption).

Since the code that handles the consolidated fields is just a post processor for definition lists, we should make sur it does not fail when hitting several terms in the same item.

[tristanlatr]

After a quick review of the code that processes definitions lists, I believe it could cause a undefined behavior when there are two terms and no classifier: more specifically I think the second term could be interpreted as being the type of the argument when should instead fail to parse the line (the type should only be specified with the classifier).