Update JSON syntax: detect nested documents via schema

When sending JSON formatted documents to Solr, particularly to /update/json/commands instead of /update/json/docs (those are API v2 paths), it tries to differentiate between wether a nested structure is either a nested document or an atomic update – it's rather ambiguous. Presently the logic is simply checking for the presence of an "id" but it may not be there (it is auto-computed when absent later). It ought to simply look in the schema to see if the field exists or not. If it doesn't, then it can't be an atomic update, thus treat it as a nested document.

This was raised on this comment in another JIRA issue.