Test Case Importer should not validate the Issue Type field

Scenario:

• The screen of Test issue creation has the field 'Issue Type'
  
• 'Issue Type' field is a required field

After the Fields Mapping configuration in the Test Case Importer, an error message appears telling that the 'Issue Type' field is missing. It should not happen since the 'Issue Type' is implicit (Test issue).

This happens with the Manual Test Step field as well. If this field is required, the Test Case Importer displays the error message.