CSV update changes Issue Type to default when not defined in import file

Steps to reproduce:

1. create Issues in JIRA;
2. create a CSV file with Issue Keys of existing Issues and defined values for other fields, but not Issue Type;
3. import the CSV in order to update existing Issues.

Expected result:
Only values included in the CSV file are updated.

Actual result:
Issue Type for all updated Issues is changed to the default one. This may result in errors e.g. in case of Workflow discrepancies.

Workaround: add Issue Type column to the CSV file and save the original values in it, to avoid overwriting.