[FIX] Issues when exporting workflows

[matteomancini]

Summary

Fixes #3230 .

List of changes proposed in this PR (pull-request)

• Removed line that skips the field parameter during export:
  

  nipype/nipype/pipeline/engine/utils.py

  Line 368 in 5116ee2

  args = args[1:]

• Added an if statement to handle strings and lists during export.

Acknowledgment

• I acknowledge that this contribution will be available under the Apache 2 license.