JIRA macro fails to display when the instance url has trailing slash

I managed to reproduce the issue on XWiki v14.10.21 and 15.10

Steps to reproduce:

1. Go to Administer Wiki > Other > JIRA and add a JIRA instance. For ex: Id: JIRA1; URL: https://jira.xwiki.org/
2. Click 'Save'
3. Go to a page and insert a JIRA macro, e.g.

{{jira id="JIRA1"}}
XWIKI-2484
XE-291
XWIKI-3677
{{/jira}}

Expected results:
The macro displays a list of 3 tickets

Actual results:
An error is displayed