RFR: 1603: Make labels handling consistent in all Issue implementations

[Zhao Song]

On Tue, 4 Oct 2022 14:10:53 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

>> Make TestIssue and JiraIssue update the cache when updating labels.
>
> test/src/main/java/org/openjdk/skara/test/TestIssue.java line 200:
> 
>> 198:     @Override
>> 199:     public void setLabels(List<String> labels) {
>> 200:         this.labels = null;
> 
> This method should update the `labels` field instead of setting it to null.

The same with JiraIssue

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PR: https://git.openjdk.org/skara/pull/1387