TypeError when Saving AR results

[xispa]

Description of the issue/feature this PR addresses

In some instances (maybe with outdated dependencies?), the exception TypeError: can only compare to a set (see below) is thrown when the a set type is used instead of list on catalog searches.

Linked issue: #812

Current behavior before PR

Traceback (innermost last):
Module ZPublisher.Publish, line 138, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 48, in call_object
Module bika.lims.browser.analysisrequest.workflow, line 64, in _call_
Module bika.lims.browser.analysisrequest.workflow, line 342, in workflow_action_submit
Module bika.lims.browser.analyses.workflow, line 219, in workflow_action_submit
Module bika.lims.api, line 655, in search
Module Products.CMFPlone.CatalogTool, line 389, in searchResults
Module Products.ZCatalog.ZCatalog, line 604, in searchResults
Module Products.ZCatalog.Catalog, line 925, in searchResults
Module Products.ZCatalog.Catalog, line 545, in search
Module Products.PluginIndexes.common.UnIndex, line 403, in _apply_index
TypeError: can only compare to a set

Desired behavior after PR is merged

Search is performed correctly without complains

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.

[ramonski]

Thanks Jordi!