[backend] Labels with + characters cannot be deleted.

Issue Summary

Labels containing + characters are not able to be deleted.
In our internal logs we can see the character is not being HTML encoded and is therefore taken as literal for example:

Steps to Reproduce

1. Create a confluence page.
2. Add a label containing a + symbol.
3. Try to delete the label.

Expected Results

1. The label should be deleted.
2. The label should not able to created.

Actual Results

The label is created and subsequently cannot be removed via the UI and the below error is displayed:

Notes

This error is not immediately helpful, however trying to remove the label via making a DELETE call to the /wiki/rest/api/content/<contentId>/label?name=10+10 endpoint returns the below:

{
    "statusCode": 404,
    "data": {
        "authorized": false,
        "valid": true,
        "errors": [],
        "successful": false
    },
    "message": "com.atlassian.confluence.api.service.exceptions.NotFoundException: No label found with name: 10"
}

Workaround

You are able to workaround this by adjusting the URL and replacing the offending + with %2B and making the DELETE call via REST eg.

curl -X DELETE \
  -u <user_email>:<api_token> \
  -H 'Accept: application/json' \
  -H 'Accept-Encoding: gzip, deflate' \
  -H 'Content-Type: application/json' \
  'https://<siteName>.atlassian.net/wiki/rest/api/content/<contentId>/label?name=10%2B10'

It's also possible to use the following code directly on the browser's console:

 fetch(
    "https://{instance}/wiki/rest/api/content/{content_id}/label/{label_name}",
    {
        "credentials":"include",
        "headers":{
            "Content-Type": "application/json"
        },
        "method":"DELETE"
    }
).then((resp) => function(response){
    console.log(response);
});