The priority does not get updated when editing issue via CSV import

Copied from https://ecosystem.atlassian.net/browse/JIM-918

As stated in the documentation:

If not specified in your CSV file, imported issues will be given the default (i.e. first) Priority as specified in your JIRA system Defining 'Priority' Field Values. You can also create new JIRA values on-the-fly during the import process.

Therefore, if the priority of the issue is not specified in the CSV file, it should be updated if the default priority has been speicified in JIRA. The behaviour should be the same with Issue Key, where if the issue key is not specified in the CSV file, it will get updated to the default.

Steps to reproduce

1. Specify the default priority in JIRA
2. Create an issue with the priority different from the default
3. Create a CSV file with issuekey,summary
4. Import the CSV file
5. Only Issuetype gets updated but not priority field