Handle ordered factor when multiple choice question uses recoded variable names

[Haunfelder]

If variableNames are non-null, use them for factor levels, otherwise use the choiceText. It may be more appropriate to add an explicit flag to the function to not cause any issues with people relying on choiceText. From my testing, all cases where variableNames do not match choiceText causes an error in fetch_survey as explained in the issue.

Fixes #342